delta-sigma again

jlarkin · 2019-11-16T17:57:37+00:00

I'm still playing with delta-sigma A/D conversion in LT Spice. I'll be using the ADUM7703 isolated 2nd order converter, clocked at 20 MHz, and we're thinking now about decimation filters in an FPGA. Win was considering using a d-s encoder and an analog recovery filter. My filter will be digital, and it will be inside a high-power control loop. The FPGA will drive a bunch of DACs at 500 KHz each, so if our d-s filter decimates it should deliver 500K samples per second to avoid squirrely aliasing hazards. I'd prefer it not decimate at all. I find the classic sinc3 recovery filter to be intellectually (ok, emotionally) repulsive. In the model below, the recovered sine wave is pretty clean near zero volts and rattier at the peaks. I don't know if that's a real property of a d-s encoder, or some Spice numerical artifact. Side story: one of my guys was plotting the frequency redponse of our class-D amplifier and got a nice curve with some weird, radical hickeys. Turns out he was signal averaging on a digital scope to remove the switching noise from the test sine wave, and reading the scope's computed RMS voltage to plot frequency response. But with some selected number of averaging samples, you get narrow frequency bands where the averaging rolls the async switching frequency back into visibility. Version 4 SHEET 1 2236 680 WIRE 1408 -208 1360 -208 WIRE 1552 -208 1488 -208 WIRE 1584 -208 1552 -208 WIRE 1648 -208 1584 -208 WIRE 1776 -208 1728 -208 WIRE 1808 -208 1776 -208 WIRE 1840 -208 1808 -208 WIRE 1552 -160 1552 -208 WIRE 1776 -160 1776 -208 WIRE 1552 -48 1552 -96 WIRE 1776 -48 1776 -96 WIRE -32 48 -80 48 WIRE 32 48 -32 48 WIRE 176 48 112 48 WIRE 240 48 176 48 WIRE 368 48 240 48 WIRE 464 48 368 48 WIRE 640 48 560 48 WIRE 704 48 640 48 WIRE 832 48 704 48 WIRE 1088 48 832 48 WIRE 1360 48 1360 -208 WIRE 1360 48 1248 48 WIRE 1408 48 1360 48 WIRE 1552 48 1488 48 WIRE 1616 48 1552 48 WIRE 1776 48 1696 48 WIRE 1824 48 1776 48 WIRE 1856 48 1824 48 WIRE 1968 48 1936 48 WIRE...

Nov 16, 2019 41 Replies

Rate this thread:

John S 6 years ago

It is not ratty at the peaks when looking at LP2 out. So, not an artifact of Spice, maybe?

(snip spice stuff)

Vote

jlarkin 6 years ago

I think it's real. LP2 is just very slow so smooths it out.

Spice is wrong in that there's no noise in a time-domain sim. Noise will randomize the system more.

I guess I could add a noise generator somewhere.

John Larkin Highland Technology, Inc lunatic fringe electronics

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required