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I'm working on a signal generator using a high speed DAC, which has balanced current outputs (two outputs, each 0-20mA, 180 degrees out of phase). The DAC is 250MSPS, so I'd like the analog frontend to handle up to about 100MHz. The output needs to be a single-ended voltage signal, with 10Vp-p.

For the same application with slower signals (a 16MHz DAC), I've used an architecture like this:

enter image description here

A simpler variant that should still work is to use resistors to ground in place of the transimpedance amps to convert the current to a voltage for the difference amplifier.

However, all the high speed amplifiers I can find, such as the THS3202 from TI, are current feedback amplifiers. As a result, their negative input has a low impedance, which - at least as far as I know - makes them unsuitable as differential input amplifiers.

How can I convert differential current outputs to a single ended voltage at these sort of frequencies, given the seeming unavailability of high speed voltage feedback opamps?

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  • \$\begingroup\$ Usually transformers- being lazy here- what does the DAC manufacturer recommend in their application note or use in their evaluation boards? \$\endgroup\$ – Spehro Pefhany Dec 5 '14 at 11:13
  • \$\begingroup\$ @Sphero I don't want to use a transformer because I want to preserve performance of the signal generator right down to DC. I think the manufacturer expects it to be used with differential outputs rather than single-ended. \$\endgroup\$ – Nick Johnson Dec 5 '14 at 11:39
  • \$\begingroup\$ So why is low input impedance to OTAs a problem? The DAC has current outputs. And why do you need to convert both outputs? Why not just run one output through your i/V converter? \$\endgroup\$ – WhatRoughBeast Dec 5 '14 at 13:08
  • \$\begingroup\$ @WhatRoughBeast I want the output to be centered around 0v, so in order to get the DC offset correct I need to use the differential signal. \$\endgroup\$ – Nick Johnson Dec 5 '14 at 13:21
  • \$\begingroup\$ @NickJohnson - What is the lowest frequency you want this generator to put out? \$\endgroup\$ – WhatRoughBeast Dec 5 '14 at 15:04
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On reflection, the answer is to heed the advice "any time you can't find an opamp with two critical characteristics, use two opamps". A voltage feedback amplifier like the LMH6657 configured as a unity gain differential amp, followed by a high slew rate current feedback amplifier to produce the required voltage swing can together produce the result I need.

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What's wrong with something like this? The LMH6624 VBF amplifier has a 1.5GHz GBW and not-terrible DC characteristics, so it ought to be reasonably good at 100MHz even with moderate gain.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ The LMH6624's slew rate is only 350V/us, which means that the maximum output amplitude of a 100mhz sine wave would be only about 0.55 volts. I realise I didn't specify in the original question (I'll update accordingly), but I need a much higher amplitude signal than that. \$\endgroup\$ – Nick Johnson Dec 5 '14 at 13:19
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THS3217 is a one-part solution to this (but wasn't available when the OP posted in 2014!)

DC to 800-MHz, Differential-to-Single-Ended, DAC Output Amplifier

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