Delta Sigma modulator help.. - Page 1
Forum section: electronics / projects

author: Junkers
author_rank: Contributor
timestamp: March 14, 2015, 10:07:21 am
content: Wondering if anyone can help me with a project I'm currently working on. I'm trying to build the analogue component of DSM and am getting strange results. I'm using two 9 volt batteries in series as a power source with respect to a floating ground i.e. +9/-9V. However, where the voltmeter appears in the diagram is where I am measuring both positive and negative voltage (of equal magnitude) despite the use of a diode.I'm trying to condition the output signal so that it's appropriate for a FPGA (0 - 3.3V) hence the need for a polarized signal. I'm not entirely sure why this is happening but feel it may have something to do with my floating ground. Can anyone confirm/explain this for me?

author: mikerj
author_rank: Super Contributor
timestamp: March 14, 2015, 10:15:53 am
content: You haven't actually told us what either the observed behaviour is, or what you were expecting. Your circuit is just an integrator followed by a comparator; with no overall feedback it's not going to be doing any modulating.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 14, 2015, 10:27:28 am
content: Reverse breakdown voltage of an LED it usually of the order of 5V.

author: Junkers
author_rank: Contributor
timestamp: March 14, 2015, 10:38:10 am
content: Sorry, was a bit terse. I did mention what I'm observing though:Quote from: Junkers on March 14, 2015, 10:07:21 amThe modulator isn't complete because I've encountered this problem; this is the circuit thus far. The circuit needs to produce a single ended output at the voltage divider for the sake of a digital input but the diode itself isn't achieving this. Here's a model of the modulator.

author: Andy Watson
author_rank: Super Contributor
timestamp: March 14, 2015, 10:44:22 am
content: Replace the last op-amp with a comparator - something like an LM311.

author: Junkers
author_rank: Contributor
timestamp: March 15, 2015, 06:08:20 am
content: Thanks for the pointers Andy. Haven't got any comparators on hand but I now have it working with 44KHz sample rate. I'll have to pick up some tomorrow. Also found the voltage reference for the DAC was coupling with the output from the FPGA. One capacitor later and it works!

author: Junkers
author_rank: Contributor
timestamp: March 16, 2015, 02:11:25 am
content: Yeah, if this ever makes it to copper then I'll be sure to do that.DSP is new realm to me so no nothing fancy, just a windowed sinc filter. I'm just trying to figure out how to convert a 1 bit signal stream into words/integers appropriate for a FIR. I'm not sure if the 1's and 0's directly translate into +9/-9 constants and then are directly fed into the FIR or whether I should be summing the frequency of 1's relative to some sampling interval e.g. 256 1's over 512 clock cycles = 0V. Can anyone verify which technique I should be using?

author: mrflibble
author_rank: Super Contributor
timestamp: March 16, 2015, 04:07:48 am
content: There's lots of good info on the matter by Bonnie Baker from TI. Do a google search on "bonnie baker delta-sigma" and you'll find the relevant material.For the digital filter that produces the N-bit output data from the 1-bit pulse train, see this one:http://www.ti.com/lit/an/slyt438/slyt438.pdf

author: diyaudio
author_rank: Frequent Contributor
timestamp: March 16, 2015, 02:01:05 pm
content: Quote from: Junkers on March 14, 2015, 10:07:21 amSignalPath did a brilliant video on Sigma Delta Modulators in ADC`s

author: TimFox
author_rank: Super Contributor
timestamp: March 16, 2015, 02:27:25 pm
content: R3 is acting as a pull-down to the negative rail and the voltmeter reading can go positive or negative with positive current through the LED

author: Kalvin
author_rank: Super Contributor
timestamp: March 16, 2015, 02:41:15 pm
content: The voltmeter is in AC instead of DC?Too long day.

author: Junkers
author_rank: Contributor
timestamp: March 19, 2015, 10:33:13 am
content: MrFibble: Thanks for the links. I have read those and a few others but find these and most other articles either approach DSM's from either a general or theoretical perspective. None seem to address the physical conversion of a bit stream into a format compatible for a FIR. I can only assume each bit is treated as an integer (1 or 0) and fed into the FIR.In the meantime I've come across another approach which involves using a CIC filter instead. I've implemented it in software and hardware and seem to be getting good results. It's a clever approach requiring no multiplication hardware and only requires 3 integrators and 3 differentiators. I'll build it over the next couple of days and see how things go. If anyone else is interested TI have a good document covering it:http://www.ti.com/lit/an/sbaa094/sbaa094.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 19, 2015, 10:45:23 am
content: I'm amazed your simulation ran at all without a ground symbol defined. Unless one of the nets was renamed to '0' (or sometimes 'GND') and this doesn't show on the schematic. Even when it does run like that, expect the results to be... dubious.Tim