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How to use DAC7821?

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rcbuck:
I want to use the TI DAC7821 to generate a 0-5 volt sine wave output. I'm not sure I understand the data sheet. Equation 1 beneath Figure 25, page 11 appears to say multiply - by Vref. If so, and Vref is 5V, I should just get an inverted sine wave since my op amp is biased at 1/2 VDD. Correct?

Looking at Figure 28 in the data sheet I think the attached circuit will work. Is that correct or wrong?

I know I can get a voltage output DAC but I want to use the DAC7821 because of its speed capabilities.

T3sl4co1l:
Raising +in doesn't work because the output pins have to be very close to ground.

You need a negative supply, one way or another.  Either negative VREF or an inverting op-amp. :-//

Or lift the whole thing up a few volts, but then you need level shifting for all the digital signals (and probably an elevated supply too).

Tim

rcbuck:
Tim,

Not sure what you mean by the output pins have to be very close to ground?

The data sheet says Vref has a range of -15 to +15 volts. Why do you need a negative supply? What happens if Vref is +5 and there is no negative supply? What would the output of the DAC be?

Ray

Ian.M:
It looks like you may be able to shift the analog ground without needing to level shift the digital signals.  If you were to bias Iout2 at 1/2 Vcc, the external OPAMP would then give an output from Vcc/2 to (Vcc - Vref), without needing a negative supply if its RRIO.  N.B. Vref > Vcc/2.  You can fix that up to get a full 0V to Vcc swing with a x2 differential amplifier stage.

With Vref, Iout1 and Rfb disconnected try biassing Iout2 at 2.5V via a 100K series resistor to limit the Iout2 pin current.   Measure the voltage drop across the resistor - it should be under 20mV.   If its not, stop - my idea is no good.

Assuming Iout2 drew negligible current with the other R-2R ladder pins disconnected, you can remove the 100K resistor, reconnect the other pins, connect Iout2 direct to the output of a rail splitter (or other low impedance Vcc/2 supply) + use that point as an elevated analog ground.  N.B. the effective Vref is relative to the potential of Iout2.

bson:
You'd need to connect Iout2 to 2.5V as well, and it needs to be a four quadrant source such as an op amp buffer.

(Edit: Ian.M beat me to it.)

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