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conditioning a fast current-out DAC

Fast DACs usually have differential current outputs. Properly converting that to a clean single-ended signal is a little tricky.

This looks good:

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The AD8130 is a cool amp. It has two differential inputs and the output is the sum of the two diff inputs. (One gotcha is that the inputs are only linear up to 2 volts or so.)

I need maybe 2.5 MHz overall bandwidth, so I started with a 3-pole

2.75 MHz Bessel filter. Then I added C1 to gobble DAC glitches and add another lowpass pole, which necessitated tweaking the LC filter a bit. This case is simple enough that a little guesswork and Spicing got the step response back to looking nice.

This is the situation where we'll grossly over-clock the DAC, 120 MHz maybe, and dither the data to improve resolution and linearity.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin
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It looks like a PI filter to me. Are PI and Bessel filters the same?

Reply to
John S

Pi is the topology. Bessel is the frequency/step response.

Single-ended LC lowpass filters are usually of the form

llll---llll---llll C C C C

and can have all sorts of different responses, depending on the values.

Bessel is good for waveform generators. It has a clean step response, no overshoot or ringing.

We'll be doing a bunch of digital filtering too, before the data gets to the DACs.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

Yes, it's an interesting design concept, with two cancelling gm inputs, one can do nice things. We feature it doing cool stuff in AoE III, e.g. see Figure 5.71, page 353. And it has excellent CMRR, Table 5.7 directs you to trace 12 in Figure 5.73.

But gm stages suffer from an intrinsically high distortion, made acceptable only by the cancelling action. Not a problem for many applications, like the CAT5 cable equalizer in our figure.

There's an amplifier type that's better: the fully- differential amplifiers, Chapter 5, section 5.17, and Table 5.10 Many of these have distortion below -80dB and to even -105dB at 10MHz, see Figure 5.106. Compare this to -74dB for the AD8130, at 1V in.

Many will happily take your DAC's current outputs directly at their differential summing junctions, arguably a better way to deal with current DACs.

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 Thanks, 
    - Win
Reply to
Winfield Hill

-74 is fine for my arb. It will be used mostly for simulating signals from rotating machines. The output amp will have more THD than the

8130.

I don't want differential gain, I want clean diff-to-single-ended conversion. We've done that with fast opamps, but the AD8130 circuit has low parts count and it's symmetric and elegant. Lots of 100 ohm resistors and 150 pF caps. It's sort of a sport here to keep the BOM line count down, to minimize pick-and-place feeders.

I may as well make the output resistors 100r 0805s like the rest of the 100's. An 0805 with good copper pours will dissipate as much heat as a 1206.

--

John Larkin         Highland Technology, Inc 

lunatic fringe electronics
Reply to
John Larkin

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