I want to convert differential-current output of DAC2902 to single-ended voltage. Can I do it with following differential op-amp configuration at the output of the DAC?

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The voltage gain is fixed = 1 in this circuit. If I need to make the output voltage gain adjustable then how can that be done?


2 Answers 2


Can I do it with following differential op-amp configuration at the output of the DAC?

If that configuration looks like figure 4 of the DAC datasheet, then yes.

(hint: it isn't. You want to have a defined, power-matched termination to ground; remember, you're not using a 125 MSPS DAC to produce DC; so you have to work like someone who has high-speed analog, not just a nearly constant signal.)

But: the AD712 has maybe 3 MHz of gain-bandwidth product. You're feeding that with a 125 MSPS DAC. Either you've chosen a DAC that's totally overkill for your application, or the opamp is far far far too slow.

I'd simply go and read the datasheet of the opamp recommended in fig. 4. It's not a random recommendation.

I need to make the output voltage gain adjustable; how can that be done?

By using a variable gain amplifier (VGA) or programmable gain amplifier (PGA). These are meant for exactly this use case, adjustable amplification.

But honestly, if you have a DAC and want to adjust the amplitude of the output, do it in the DAC.

Your DAC even has a dedicated gain setting functionality. So, really, read the DAC datasheet from front to back until you figure out how to use that.

If that isn't fine enough (or large enough) in dynamic range, you could also consider scaling the digital values.

If it can be avoided, don't try to multiply analog things; that is very hard to do accurately. That's why DACs are a bit more expensive than they'd be if they were just a bunch of resistors and transistor switches thrown in an IC. Let the DAC do your analog signal scaling – it's what you've bought it for.


Current_output DACs will have a "voltage compliance range" as part of the datasheet.

To achieve very high speed (which has changed over the many decades from 1uS to 100 picosecond), the DAC internal current switches need a very low resistance/impedance, to minimize the Miller_Effect in the switches.

At the highest speed, you use common_base circuits, operating at DAC_Imax/2 so the external transistor is always ON and operating quickly.

WIth dual outputs, use 2 common_base circuits, emitter resistor to -5v (this will be a determinist noise input, from RAIL trash. Tie an appropriate collector resistor (assuming NPN device) to +5v, which is also a determinist noise input.

Now, with 2 bipolar collector outputs, perhaps sitting at +4v without any DAC current, the two collector voltages will only drop, closer to Ground, as the DAC is connected and walked thru its Codes.

You have differential voltage, with some common_mode voltage.

Use a standard Differential_mode to Single_ended opamp circuit.

If you want the fastest speed, then use two current_mirrors, subtract the currents, provide the output current into a resistor to Ground. This will not be as exact, but easily preserves nanosecond edges.


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