trying to understand the REF pin on a DAC

Guest
Can someone explain what the REF pin on my DAC is for?
It's an input voltage reference that can be GND, Vdd-1.5 or nominally
2.048v at Vdd =5v.

What is this voltage used for?
How does it affect the output?

Some of the circuits I see have tied this to Vdd - which seems too
high.

(The chip is the 5618 12-bit serial DAC)

Thanks,
Dave
 
kasterborus@yahoo.com wrote:
Can someone explain what the REF pin on my DAC is for?
It's an input voltage reference that can be GND, Vdd-1.5 or nominally
2.048v at Vdd =5v.

What is this voltage used for?
How does it affect the output?

Some of the circuits I see have tied this to Vdd - which seems too
high.

(The chip is the 5618 12-bit serial DAC)
Click to expand...
The ref pin generally scales the full scale output of the
DAC. If you think of the digital value going into the DAC
as a fraction that goes from zero to (2^12)/((2^12)-1), the
output voltage will be that fraction multiplied by the
reference voltage. In effect, the DAC is a one quadrant
multiplier that that multiplies an analog voltage (the ref)
by a digital fraction.
 
John Popelish wrote:
kasterborus@yahoo.com wrote:
Can someone explain what the REF pin on my DAC is for?
It's an input voltage reference that can be GND, Vdd-1.5 or nominally
2.048v at Vdd =5v.

What is this voltage used for?
How does it affect the output?

Some of the circuits I see have tied this to Vdd - which seems too
high.

(The chip is the 5618 12-bit serial DAC)

The ref pin generally scales the full scale output of the DAC. If you
think of the digital value going into the DAC as a fraction that goes
from zero to (2^12)/((2^12)-1), the output voltage will be that fraction
multiplied by the reference voltage. In effect, the DAC is a one
quadrant multiplier that that multiplies an analog voltage (the ref) by
a digital fraction.
Click to expand...
Sorry, that should have read
".../((2^12)+1)"
 
On Mon, 29 Oct 2007 15:51:45 -0000, kasterborus@yahoo.com wrote:

Can someone explain what the REF pin on my DAC is for?
It's an input voltage reference that can be GND, Vdd-1.5 or nominally
2.048v at Vdd =5v.

What is this voltage used for?
How does it affect the output?

Some of the circuits I see have tied this to Vdd - which seems too
high.

(The chip is the 5618 12-bit serial DAC)

Thanks,
Dave
Click to expand...

The output voltage is

Vout = YourDigitalCode * Vref / 4096

so it directly affects the output.

John
 
John Popelish wrote:
John Popelish wrote:
kasterborus@yahoo.com wrote:
Can someone explain what the REF pin on my DAC is for?
It's an input voltage reference that can be GND, Vdd-1.5 or nominally
2.048v at Vdd =5v.

What is this voltage used for?
How does it affect the output?

Some of the circuits I see have tied this to Vdd - which seems too
high.

(The chip is the 5618 12-bit serial DAC)

The ref pin generally scales the full scale output of the DAC. If you
think of the digital value going into the DAC as a fraction that goes
from zero to (2^12)/((2^12)-1), the output voltage will be that
fraction multiplied by the reference voltage. In effect, the DAC is a
one quadrant multiplier that that multiplies an analog voltage (the
ref) by a digital fraction.

Sorry, that should have read
".../((2^12)+1)"
Click to expand...
Arggg! Make that "((2^12)-1)/(2^12). The largest value the
digital input can have for a 12 bit converter is (2^12)-1,
but the converter has 2^12th states, one of those being zero.
 
kasterborus@yahoo.com wrote:

Can someone explain what the REF pin on my DAC is for?
Click to expand...
It sets the 'full scale deflection' of the DAC. Very useful in some instances.

In other instances it may be used to filter/smooth the reference voltage so that
it has very low noise.

Graham
 
Wow thanks - that makes sense...

I was reading the datasheet for the chip and I have a question...
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

At one point it says that the maximum ref voltage is Vdd + 0.3, then
later is says the maximum recommended Vref is Vdd-1.5.
Would it be OK just to tie this pin to the 5v I'm using to power the
chip?

I want the output to be 0-5v.

Dave
 
kasterborus@yahoo.com wrote:

Wow thanks - that makes sense...

I was reading the datasheet for the chip and I have a question...
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

At one point it says that the maximum ref voltage is Vdd + 0.3,
Click to expand...
Yes, that's the 'absolute maximum' above which damage will occur.


then later is says the maximum recommended Vref is Vdd-1.5.
Click to expand...
For reliable performance no doubt.


Would it be OK just to tie this pin to the 5v I'm using to power the
chip?
Click to expand...
Best to filter it first. Even with just an RC.

Graham
 
On Mon, 29 Oct 2007 17:53:10 -0000, kasterborus@yahoo.com wrote:

Wow thanks - that makes sense...

I was reading the datasheet for the chip and I have a question...
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

At one point it says that the maximum ref voltage is Vdd + 0.3, then
later is says the maximum recommended Vref is Vdd-1.5.
Would it be OK just to tie this pin to the 5v I'm using to power the
chip?
Click to expand...
---
No.
---

I want the output to be 0-5v.
Click to expand...
---
Look at the top of page 11 of the data sheet and you'll see:

"The output voltage (full scale determined by the reference) is
given by:

CODE
2 REF ------ [V]"
2^n

which means that if your code is hex fff and 2^n = 4096, (which it
does since it's a 12 bitDAC) then with a reference voltage of 2.5V
your output will go from zero volts with an input of 0x000 to 5V
with an input of 0xfff.


--
JF
 
On Mon, 29 Oct 2007 15:17:00 -0500, John Fields
<jfields@austininstruments.com> wrote:

Oops..,

---
Look at the top of page 11 of the data sheet and you'll see:
^^
Click to expand...
10

--
JF
 
kasterborus@yahoo.com wrote:
Wow thanks - that makes sense...

I was reading the datasheet for the chip and I have a question...
http://focus.ti.com/lit/ds/symlink/tlv5618a.pdf

At one point it says that the maximum ref voltage is Vdd + 0.3, then
later is says the maximum recommended Vref is Vdd-1.5.
Would it be OK just to tie this pin to the 5v I'm using to power the
chip?

I want the output to be 0-5v.
Click to expand...
You should have supplied the link to the data sheet the
first time. ;-)

See the note under the recommended operating conditions
block on page 3.

This beast has times 2 amplifiers between the ref input and
the DAC, and, evidently (though they don't seem so proud of
the fact, hence the fine print) those amplifiers cannot
swing anywhere near Vdd, so the maximum ref input that will
not cause clipping the high end digital values is Vdd-0.4/2.

For a 5 volt Vdd, that is 2.3 volts, for an internal
reference voltage of 4.6 volts, and the output cannot exceed
that.

You would need to power this chip from at least 5.4 volts to
get a 5 volt, full scale output, with a 2.5 volt reference
input.

This is the first DAC I have seen with this internal
reference multiplier "feature". If they were proud of this
feature they might have written a 2X inside the pair of
triangles on the block diagram at the top of page 2, with
the note about the output limitation of the amplifiers,
there, also.
 
Wow - thanks for all the responses, I've been setting the REF to 2.048
(or as close as I can get) using a pot. The DAC appears to be
functioning, but the output is not as expected.

1100000000000000
0

1100001111101000
0

1100011111010000
483

1100111110100000
484

Decimal is the input pin of an Arduino "analog in" - I'm not sure if
full scale is 500, or 1000, but as you can see the DAC is not
returning analog voltages quite in step with the input commands.

I'm plugging the chip directly into the Arduino board, and measuring
the output as soon as the CS pin goes back high.

Is is OK for me to do this, or do I need resistors and capacitors in
the mix too?

Please forgive my ignorance with this - It's part of a larger software
(my forte) project, and I do struggle a bit with the electronics.
 
On Tue, 30 Oct 2007 13:50:42 -0000, kasterborus@yahoo.com wrote:


Decimal is the input pin of an Arduino "analog in" - I'm not sure if
full scale is 500, or 1000, but as you can see the DAC is not
returning analog voltages quite in step with the input commands.
Click to expand...
If you are unsure of the Arduino ADC characteristic (and because your
software reading the ADC may have problems), you should measure the
DAC output with a voltmeter - that should give you a clear indication
of what the DAC is doing, without the added confusion of having to
read the ADC.

To confirm operation of the ADC, you could apply a variable voltage to
the ADC input, and see how the readings vary as you vary the voltage.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca
 
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