Shunt Reference Problems

M

Michael Kellett

Guest
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The 4
PSU outputs are also monitored by a Keysight 34972A outside the chamber.
I’m not bothered by the actual output voltages – these are dependent on
the cheap and cheerful LM4040s and within spec (although I don’t like
the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured
(by 34972A) outputs with the self-reported output based on the ADR530B
references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the problem,
so I replaced the resistors with expensive ones and added another 4 x
34972A channels to monitor the ADR530Bs directly.

This got me psu_plot_refs_ltcrs_1.jpg.

There’s still some bad stuff going on with the blue 15V channel – but I
don’t care about that yet – concentrate on the last plot – that’s the 4
ADR530B chips monitored directly, the temperature coefficients over the
5- 60C range work out at:

65, 56, 53, 53 ppm/C.

Well, it could still be my ADC or some other thing I’ve missed in my
design, so I built a little test board with 2 x Microchip LM4040 and 2 x
Analog ADR530B, there’s a picture of it set up in the temperature cycler.

Each ref chip is connected to a common 10V supply by a 2k2 resistor.
Each ref chip is shunted with a 470nF cap.
The results are shown in ref_chips_1.jpg

The temperature coefficients over the 5- 60C range work out at:

-27, 46, 50, 54 ppm/C.

The Microchip parts are within spec but the one with the blue trace has
a seriously odd behaviour (this is NOT the same channel on the 34972A as
the previous “bumpy” LM4040).

The real problem is the ADR530B parts – perfectly linear and smooth
tracking of temperature, and out of spec.

I’ll be following this up with AD (of course) but I’d be interested in
any comments.

MK
 
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk>
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.
Click to expand...

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
On Sunday, February 16, 2020 at 9:51:24 AM UTC-5, Michael Kellett wrote:

Only your first link is working.
At least on my reader.

The 15V output (blue trace) in psu_plot_1 does seem to have some quirky behavior, but it may not be related to the problem ADR530B.

Have you tried this running a precision reference for the (Microchip) ADC?
 
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk>
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The 4
PSU outputs are also monitored by a Keysight 34972A outside the chamber.
I’m not bothered by the actual output voltages – these are dependent on
the cheap and cheerful LM4040s and within spec (although I don’t like
the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured
(by 34972A) outputs with the self-reported output based on the ADR530B
references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the problem,
so I replaced the resistors with expensive ones and added another 4 x
34972A channels to monitor the ADR530Bs directly.

This got me psu_plot_refs_ltcrs_1.jpg.

There’s still some bad stuff going on with the blue 15V channel – but I
don’t care about that yet – concentrate on the last plot – that’s the 4
ADR530B chips monitored directly, the temperature coefficients over the
5- 60C range work out at:

65, 56, 53, 53 ppm/C.

Well, it could still be my ADC or some other thing I’ve missed in my
design, so I built a little test board with 2 x Microchip LM4040 and 2 x
Analog ADR530B, there’s a picture of it set up in the temperature cycler.

Each ref chip is connected to a common 10V supply by a 2k2 resistor.
Each ref chip is shunted with a 470nF cap.
The results are shown in ref_chips_1.jpg

The temperature coefficients over the 5- 60C range work out at:

-27, 46, 50, 54 ppm/C.

The Microchip parts are within spec but the one with the blue trace has
a seriously odd behaviour (this is NOT the same channel on the 34972A as
the previous “bumpy” LM4040).

The real problem is the ADR530B parts – perfectly linear and smooth
tracking of temperature, and out of spec.

I’ll be following this up with AD (of course) but I’d be interested in
any comments.

MK
Click to expand...

How fast are you sampling and is this a low-side (near ground) shunt
current sense ?

If slow-ish sample rate, you could use that Microchip 16 bit
differential input A/D that does a zero offset between every sample.
Higher sample rates at lower resolution I think. Works really well
for my current sense applications. It is also I2C though which may or
may not be an issue for you.
 
On 2/16/20 9:51 AM, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested  “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The 4
PSU outputs are also monitored by a Keysight 34972A outside the chamber.
I’m not bothered by the actual output voltages – these are dependent on
the cheap and cheerful LM4040s and within spec (although I don’t like
the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured
(by 34972A) outputs with the self-reported output based on the ADR530B
references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the problem,
so I replaced the resistors with expensive ones and added another 4 x
34972A channels to monitor the ADR530Bs directly.
Click to expand...

What kind of "expensive resistors"? Do they have a low enough absolute
tempco that even if the tempcos in the divider network conspire against
you (there's no law they have to track together if they weren't matched
for that purpose...) it's not enough to explain the observed error?

<https://www.analog.com/media/en/technical-documentation/data-sheets/5400fc.pdf>
 
On Sunday, February 16, 2020 at 6:51:24 AM UTC-8, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.
Click to expand...

If it really IS the ADR530(s), your best bet is to consider a thermal block
(testing 0-80C, maybe heat to 90C?) for the sensitive parts. A little fluff,
a can, and a thermostat will make the thermal drifts irrelevant. And,
you won't need the -B version. But, it's a nuisance at build time, and
there's always the warmup period.
 
On 16/02/2020 22:54, whit3rd wrote:
On Sunday, February 16, 2020 at 6:51:24 AM UTC-8, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

If it really IS the ADR530(s), your best bet is to consider a thermal block
(testing 0-80C, maybe heat to 90C?) for the sensitive parts. A little fluff,
a can, and a thermostat will make the thermal drifts irrelevant. And,
you won't need the -B version. But, it's a nuisance at build time, and
there's always the warmup period.
Click to expand...
At least you've got the point - the error does come from the AD530B parts.
The problem with your suggestion is that the available space, power and
cost won't support it. (There are 4 isolated channels and each needs its
own reference and ADC.)
What's bugging me is not that the voltage references are not perfect but
that they do not meet their own spec.
Thanks for the suggestion.

MK
 
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk>
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK
Click to expand...

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
On 16/02/2020 22:44, bitrex wrote:
On 2/16/20 9:51 AM, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and
isolated channels which use Microchip LM4040 2.5V references. It also
has opto isolated readback for each channel which uses Analog ADR530B
references and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested  “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The
4 PSU outputs are also monitored by a Keysight 34972A outside the
chamber.
I’m not bothered by the actual output voltages – these are dependent
on the cheap and cheerful LM4040s and within spec (although I don’t
like the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the
measured (by 34972A) outputs with the self-reported output based on
the ADR530B references and the temperature drift is much higher than
40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the
problem, so I replaced the resistors with expensive ones and added
another 4 x 34972A channels to monitor the ADR530Bs directly.

What kind of "expensive resistors"? Do they have a low enough absolute
tempco that even if the tempcos in the divider network conspire against
you (there's no law they have to track together if they weren't matched
for that purpose...) it's not enough to explain the observed error?

https://www.analog.com/media/en/technical-documentation/data-sheets/5400fc.pdf
Click to expand...
It doesn't matter what the resistors are or were - as I explained I
'added another 4 x 34972A channels to monitor the ADR530Bs directly' and
when I did that they don't meet spec, and they don't meet spec on a test
board either.

MK
 
On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.
Click to expand...
Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK
 
On 16/02/2020 21:42, boB wrote:

If slow-ish sample rate, you could use that Microchip 16 bit
differential input A/D that does a zero offset between every sample.
Higher sample rates at lower resolution I think. Works really well
for my current sense applications. It is also I2C though which may or
may not be an issue for you.
Click to expand...

Thanks for the suggestion but it isn't the ADC that is the problem -
it's the ADR530B voltage reference - see my reply to JL below.

I have had to fit the entire isolated voltage reference and ADC onto a
tiny board that replaces an 8 pin SOIC part. If I respin the whole board
other solutions are possible - but currently I'm more bothered by the
implications of the ADR530B not meeting pec.

MK
 
On 16/02/2020 19:59, mpm wrote:
On Sunday, February 16, 2020 at 9:51:24 AM UTC-5, Michael Kellett wrote:

Only your first link is working.
At least on my reader.

The 15V output (blue trace) in psu_plot_1 does seem to have some quirky behavior, but it may not be related to the problem ADR530B.

Have you tried this running a precision reference for the (Microchip) ADC?

The dropbox link should get you all the diagrams - it does on Firefox.
Click to expand...

The Microchip voltage references do have their own issues but they do
meet spec. I'm not yet prepared to swear that the steppiness and other
quirks are definately down them.

My big problem is that the top grade ADR530B doesn't meet its spec.

MK
 
On Sunday, 16 February 2020 09:51:24 UTC-5, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The 4
PSU outputs are also monitored by a Keysight 34972A outside the chamber.
I’m not bothered by the actual output voltages – these are dependent on
the cheap and cheerful LM4040s and within spec (although I don’t like
the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured
(by 34972A) outputs with the self-reported output based on the ADR530B
references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the problem,
so I replaced the resistors with expensive ones and added another 4 x
34972A channels to monitor the ADR530Bs directly.

This got me psu_plot_refs_ltcrs_1.jpg.

There’s still some bad stuff going on with the blue 15V channel – but I
don’t care about that yet – concentrate on the last plot – that’s the 4
ADR530B chips monitored directly, the temperature coefficients over the
5- 60C range work out at:

65, 56, 53, 53 ppm/C.

Well, it could still be my ADC or some other thing I’ve missed in my
design, so I built a little test board with 2 x Microchip LM4040 and 2 x
Analog ADR530B, there’s a picture of it set up in the temperature cycler.

Each ref chip is connected to a common 10V supply by a 2k2 resistor.
Each ref chip is shunted with a 470nF cap.
The results are shown in ref_chips_1.jpg

The temperature coefficients over the 5- 60C range work out at:

-27, 46, 50, 54 ppm/C.

The Microchip parts are within spec but the one with the blue trace has
a seriously odd behaviour (this is NOT the same channel on the 34972A as
the previous “bumpy” LM4040).

The real problem is the ADR530B parts – perfectly linear and smooth
tracking of temperature, and out of spec.

I’ll be following this up with AD (of course) but I’d be interested in
any comments.

MK
Click to expand...

In your tests to attempt to isolate the reference- did you have nothing else in the environmental chamber? If the power supply was there, as it appeared to be in the photo, was it operating during the test?

Best regards,
Spehro Pefhany
 
On 17/02/2020 11:04, speff wrote:
On Sunday, 16 February 2020 09:51:24 UTC-5, Michael Kellett wrote:
I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

The ADR530B Temp Coefficient is specified as 15ppm typical and 40ppm
max. There is a note saying “Guaranteed by design, but not production
tested “ .

When I ran it though a temperature cycle, I was a bit disappointed to
get the results in psu_plot_1.jpg.

https://www.dropbox.com/sh/8f6b34u58gg400r/AABcRqFMQ-Q0JAVOAzFRcxJra?dl=0

For the temperature tests the power supply runs off the mains, its on
board processor reports results to a PC outside the test chamber. The 4
PSU outputs are also monitored by a Keysight 34972A outside the chamber.
I’m not bothered by the actual output voltages – these are dependent on
the cheap and cheerful LM4040s and within spec (although I don’t like
the steps on the blue 15V channel).

The 15 and 5 V measurement errors plots come from comparing the measured
(by 34972A) outputs with the self-reported output based on the ADR530B
references and the temperature drift is much higher than 40ppm.

My first thought was that external parts, probably voltage divider
resistors or maybe even the tiny processor ADC was causing the problem,
so I replaced the resistors with expensive ones and added another 4 x
34972A channels to monitor the ADR530Bs directly.

This got me psu_plot_refs_ltcrs_1.jpg.

There’s still some bad stuff going on with the blue 15V channel – but I
don’t care about that yet – concentrate on the last plot – that’s the 4
ADR530B chips monitored directly, the temperature coefficients over the
5- 60C range work out at:

65, 56, 53, 53 ppm/C.

Well, it could still be my ADC or some other thing I’ve missed in my
design, so I built a little test board with 2 x Microchip LM4040 and 2 x
Analog ADR530B, there’s a picture of it set up in the temperature cycler.

Each ref chip is connected to a common 10V supply by a 2k2 resistor.
Each ref chip is shunted with a 470nF cap.
The results are shown in ref_chips_1.jpg

The temperature coefficients over the 5- 60C range work out at:

-27, 46, 50, 54 ppm/C.

The Microchip parts are within spec but the one with the blue trace has
a seriously odd behaviour (this is NOT the same channel on the 34972A as
the previous “bumpy” LM4040).

The real problem is the ADR530B parts – perfectly linear and smooth
tracking of temperature, and out of spec.

I’ll be following this up with AD (of course) but I’d be interested in
any comments.

MK

In your tests to attempt to isolate the reference- did you have nothing else in the environmental chamber? If the power supply was there, as it appeared to be in the photo, was it operating during the test?

Best regards,
Spehro Pefhany
Click to expand...
Good point, thanks - it was there but not plugged in - if you look very
carefully you can see the unnconected IEC mains connector at the bottom
left (and it was unconnected during the test - I checked the testfile
and all the PSU signals are at zero).


I've ordered some similar ADI parts (ADR525B) and I'll add those to the
test board, and maybe a few others - the 34972A has lots of channels !

MK
 
On 17/02/2020 09:41, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.
Click to expand...
I wouldn't expect ADI to ship bad parts either - which is why I raise
the subject at all.

The test board setup is so simple that it's rather hard to imagine what
might be wrong with it. Yet the ADR530B parts had beattifully linear but
out of spec temperaure coefficients. And the Microchip LM4040 parts had
rather quirky (but in spec) behaviour.

I had hoped that someone on SED might have experience of good or bad
behaviour of similar shunt references.

I've ordered some ADR250B parts and I'll add them to the same test and
repeat.

MK
 
On Mon, 17 Feb 2020 11:12:05 +0000, Michael Kellett <mk@mkesc.co.uk>
wrote:

On 17/02/2020 09:41, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.



I wouldn't expect ADI to ship bad parts either - which is why I raise
the subject at all.

The test board setup is so simple that it's rather hard to imagine what
might be wrong with it. Yet the ADR530B parts had beattifully linear but
out of spec temperaure coefficients. And the Microchip LM4040 parts had
rather quirky (but in spec) behaviour.

I had hoped that someone on SED might have experience of good or bad
behaviour of similar shunt references.

I've ordered some ADR250B parts and I'll add them to the same test and
repeat.

MK
Click to expand...

Be careful about EMI. We are in a high EMI location, and we see all
sorts of strange rectifying effects.

I do wonder how they can guarantee tempco by design not testing, but
sell different tempco grades. Maybe they assume that voltage accuracy
implies tempco. Or maybe all the parts are the same but just the
prices are different.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
On 2/17/20 4:41 AM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.
Click to expand...

With reference back to the "two engineering fundamentals" before you
could begin to assume that it's the parts and not a setup problem have
to show that your measurement setup can in practice, not just in theory,
measure a device with a 40ppm/C or better tempco operating to its
manufacturer's spec in the first place.

Otherwise what justification is there to j'accuse the butler when the
maid doesn't even have an airtight alibi yet.
 
On 2020-02-17 04:41, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.
Click to expand...

IIRC datasheet temperature coefficient isn't dV/dT, it's Delta V (max
P-P deviation over the specified temperature range)/Delta T.

dV/dT is allowed to be much larger in smaller ranges. (Offset drift of
amplifiers is specified the same way.)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com
 
On Mon, 17 Feb 2020 11:34:52 -0500, bitrex <user@example.net> wrote:

On 2/17/20 4:41 AM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.



With reference back to the "two engineering fundamentals" before you
could begin to assume that it's the parts and not a setup problem have
to show that your measurement setup can in practice, not just in theory,
measure a device with a 40ppm/C or better tempco operating to its
manufacturer's spec in the first place.

Otherwise what justification is there to j'accuse the butler when the
maid doesn't even have an airtight alibi yet.
Click to expand...

Right. Stop and think.



--

John Larkin Highland Technology, Inc

The cork popped merrily, and Lord Peter rose to his feet.
"Bunter", he said, "I give you a toast. The triumph of Instinct over Reason"
 
On 17/02/2020 16:34, bitrex wrote:
On 2/17/20 4:41 AM, jlarkin@highlandsniptechnology.com wrote:
On Mon, 17 Feb 2020 09:27:59 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

On 16/02/2020 20:37, jlarkin@highlandsniptechnology.com wrote:
On Sun, 16 Feb 2020 14:51:21 +0000, Michael Kellett <mk@mkesc.co.uk
wrote:

I’m working on a power supply design. It has 4 independent and
isolated
channels which use Microchip LM4040 2.5V references. It also has opto
isolated readback for each channel which uses Analog ADR530B
references
and a tiny ST micro.

Does that mean that you are using the ADC in the micro with the ADR as
its reference? Those kinds of ADCs aren't especially accurate, no
matter how good the reference.



Actually the ADC is pretty good, the problem is the ADR530B, as is
proved by measuring it directly both in the complete PSU and on a test
board. Of 6 tested none meet the specified 40ppm/C mac tempco.

MK

I don't think ADI is shipping bad parts. Sounds like some kind of
setup problem.



With reference back to the "two engineering fundamentals" before you
could begin to assume that it's the parts and not a setup problem have
to show that your measurement setup can in practice, not just in theory,
measure a device with a 40ppm/C or better tempco operating to its
manufacturer's spec in the first place.

Otherwise what justification is there to j'accuse the butler when the
maid doesn't even have an airtight alibi yet.
Click to expand...
Well, there is some evidence that the test rig is OK because one of the
Microchip parts does better than 40ppm.
The nature of the test rig suggests that it is likely to work correctly
( the measuring device and power supply are outside the temperature cycler).
I too would like to see a part actually achieve the spec under test so
I've ordered some 2.5V parts. I would have liked to try some ADR530Bs
from a different supplier but no one local had them in stock.

I've got a couple of LTC6652AHMS8-4.096#PBF (5ppm) kicking around so
I'll try to graft them onto the test board as well.

MK
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top