LPC1768 ADC

[John Larkin]

Does anyone use the LPC176* uPs?

I need to know how much current the ADC/DAC VREFP pin uses, and I
can't find that anywhere. You'd think they might spec that.

They recommend filtering it, but the eval board uses a ferrite bead.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Lasse Langwadt Christense]

torsdag den 9. januar 2020 kl. 02.06.09 UTC+1 skrev John Larkin:

Does anyone use the LPC176* uPs?

I need to know how much current the ADC/DAC VREFP pin uses, and I
can't find that anywhere. You'd think they might spec that.

They recommend filtering it, but the eval board uses a ferrite bead.

google says, https://community.nxp.com/thread/388939

"The input impedance of Vref is around 7k Ohms, so this should be taken into consideration when using a voltage divider on this input."

 

[John Larkin]

On Wed, 8 Jan 2020 17:18:35 -0800 (PST), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

torsdag den 9. januar 2020 kl. 02.06.09 UTC+1 skrev John Larkin:
Does anyone use the LPC176* uPs?

I need to know how much current the ADC/DAC VREFP pin uses, and I
can't find that anywhere. You'd think they might spec that.

They recommend filtering it, but the eval board uses a ferrite bead.


google says, https://community.nxp.com/thread/388939

"The input impedance of Vref is around 7k Ohms, so this should be taken into consideration when using a voltage divider on this input."

Thanks. I did search the NXP community posts, with various keywords,
and didn't find that one.

I can probably use a 50 ohm series resistor.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Winfield Hill]

John Larkin wrote...

I can probably use a 50 ohm series resistor.

It may need a capacitor to handle pulse currents.


--
Thanks,
- Win

 

On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com>
wrote:

John Larkin wrote...

I can probably use a 50 ohm series resistor.

It may need a capacitor to handle pulse currents.

Sure, all my voltage reference noise filters involve capacitors
somewhere.

Maybe I'll provide a separate voltage reference for the uP ADC, a
local bandgap and cap. Then I can digitize and check the *other*
reference. 3V for the uP, 2.5 for everything else.

This will be a lithium niobate EO modulator driver, with a very
expensive (over $200) distributed amplifier chip. It has a lot of
sequencing requirements and a lot of protections.





--

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"

 

[boB]

On Thu, 09 Jan 2020 08:04:22 -0800, jlarkin@highlandsniptechnology.com
wrote:

On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

John Larkin wrote...

I can probably use a 50 ohm series resistor.

It may need a capacitor to handle pulse currents.

Sure, all my voltage reference noise filters involve capacitors
somewhere.

Maybe I'll provide a separate voltage reference for the uP ADC, a
local bandgap and cap. Then I can digitize and check the *other*
reference. 3V for the uP, 2.5 for everything else.

This will be a lithium niobate EO modulator driver, with a very
expensive (over $200) distributed amplifier chip. It has a lot of
sequencing requirements and a lot of protections.

We use that series of parts. Always place some capacitance across the
A/D input to analog Vss.... Usually this is my 1st order low pass
anyway.

For Vref+ I just use a small resistance and a capacitor. 1 Ohm or a
few more does pretty well.

 

[John Larkin]

On Thu, 09 Jan 2020 13:58:05 -0800, boB <boB@K7IQ.com> wrote:

On Thu, 09 Jan 2020 08:04:22 -0800, jlarkin@highlandsniptechnology.com
wrote:

On 8 Jan 2020 18:33:32 -0800, Winfield Hill <winfieldhill@yahoo.com
wrote:

John Larkin wrote...

I can probably use a 50 ohm series resistor.

It may need a capacitor to handle pulse currents.

Sure, all my voltage reference noise filters involve capacitors
somewhere.

Maybe I'll provide a separate voltage reference for the uP ADC, a
local bandgap and cap. Then I can digitize and check the *other*
reference. 3V for the uP, 2.5 for everything else.

This will be a lithium niobate EO modulator driver, with a very
expensive (over $200) distributed amplifier chip. It has a lot of
sequencing requirements and a lot of protections.


We use that series of parts. Always place some capacitance across the
A/D input to analog Vss.... Usually this is my 1st order low pass
anyway.

For Vref+ I just use a small resistance and a capacitor. 1 Ohm or a
few more does pretty well.

OK, sounds like a few ohms, 10 or so maybe, and a pretty big ceramic
cap would be OK.

Or my dedicated 3V bandgap running at a few mA.

The NXP data sheets are weird. They specify the analog power and
reference currents in various shutdown modes, but not operating.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com