20-bit ADC measurement board design - Page 1
Forum section: electronics / projects

author: bobwidlar
author_rank: Contributor
timestamp: June 30, 2014, 04:11:39 am
content: Hi,I've been working on the test board of my 20-bit sigma-delta ADC. I think you might find it interesting, and maybe give me some feedback on the design. I'm trying to achieve better than -110dB THD+N on my measurement setup, so that the any higher error is dominated by the ADC itself.I'm not in the office now so I will post some schematics later. But basically, it's just a simple board with a fully-differential (balanced) analog input and 5-bit quantizer output. Input buffers are either LME49990 or LT1128 low-noise low-distortion OPAMPs. The voltage reference is low noise low drift LTC6655 (2.048V and has a impressively low flicker noise) which is divided to 1.8V (VDD and positive reference of the ADC) and 0.9V (common-mode). These voltages are buffered via AD8675s, and filtered a little bit. Digital is isolated, so I can directly use an NI digital IO board and not worry about PC ground noise. The board will be battery powered, since the ADC consumes only 2mW it doesn't need a mains supply.Signal source will be Audio Precision 2722. I will do some quick checks with Stanford Research DS360 low distortion function generator also. It has a low enough distortion for me but noise floor is quite high.

author: jmt
author_rank: Newbie
timestamp: June 30, 2014, 04:32:00 am
content: This sounds very interesting indeed, I look forward to some schematics and your layout guidelines. A few questions for now:Is the ADC and ASIC or do you have a part number? I'm curious what goes into a 20-bit ADC.What voltage range do you plan to be able to encode?What is the application?

author: bobwidlar
author_rank: Contributor
timestamp: June 30, 2014, 05:09:24 am
content: It's a prototype, I designed it myself. Well, I can't give the information what I have inside of this particular chip but you can ask me anything about ADCs and DACs.Input range is almost rail-to-rail fully differential. So, a little bit less than 3.6V peak-to-peak. Application is audio and instrumentation.

author: tree
author_rank: Regular Contributor
timestamp: June 30, 2014, 01:44:47 pm
content: Did you design/manufacture it using a batch IC service or for work?

author: bobwidlar
author_rank: Contributor
timestamp: June 30, 2014, 09:09:44 pm
content: It's an MPW run for work

author: tree
author_rank: Regular Contributor
timestamp: June 30, 2014, 09:12:26 pm
content: How long did it take to design it?When you begin designing an IC, do you first start with a block level schematic and then go to transistor level design? Did you design it by hand or with the help of a simulator?

author: Zad
author_rank: Super Contributor
timestamp: July 01, 2014, 01:05:58 am
content: What PCB substrate will you be using, FR4 or something more exotic? For ultimate performance it might be worth going for ceramic or PTFE.

author: mathias
author_rank: Regular Contributor
timestamp: July 01, 2014, 01:53:42 pm
content: What kind of ultimate performance? Both ceramic and PTFE are used for microwave circuits (controlled impedance and low loss). Perhaps PTFE can be useful for low leakage, but usually they use stand offs for this purpose, as PTFE board manufacturing is expensive.

author: bobwidlar
author_rank: Contributor
timestamp: July 01, 2014, 04:54:22 pm
content: Quote from: tree on June 30, 2014, 09:12:26 pmWell, more than a year to design it. I use Matlab & simulink for system level design and then Cadence for all schematic level and physical design. I used Cadence RTL compiler for digital synthesis also. These are more like the standard tools for IC design, at least for analog / mixed-signal. Hand calculations are very very very rare, since any model you can use for hand calculations not realistic.Quote from: Zad on July 01, 2014, 01:05:58 am