DAQ/Sensor Design Considerations - Page 1
Forum section: electronics / projects

author: gilbenl
author_rank: Regular Contributor
timestamp: January 12, 2016, 03:04:42 pm
content: I'm designing a DAQ system for in vivo biomedical research (non-human) applications that is intended to be a relatively low-cost substitute for currently available products. The signal path is sensor->wheatstone bridge (aka "bridge")->op amp->ADC. Fairly straightforward.I've unfortunately "thought" my way into uncertainty and wanted to run this by the masses. Specifically, how to configure the bridge/sensor (ie: What is attached vs what is built into the box). I would like to be able to plug in just about any sensor into the thing, however, for my application, it needs to be able to measure temperature or pressure in a given channel. For this example, let's consider a precision thermistor. These are the 3 configurations that I'm wrestling with and my thoughts on each:1) Thermistor hand piece with 5' shielded lead plugs into DAQ. Bridge, amp etc all built into DAQ.Pros: Easily designed/cheaper sensor attachments. 2 wire connectionCons: Sensor leads may be long-As they are effectively R3 in the bridge with the sensor, it seems the leads would be the "weakest link" with regards to tempco stability of the system. Fixed bridge resistance unless multiple selectable legs are used.2) Thermistor with corresponding bridge leg in hand piece. 5' shielded lead plugs into DAQ. Remaining half-bridge, amp, etc built into DAQ.Pros: Corresponding side of bridge will be in same thermal environment. 2 wire connectionCons: ?3) Thermistor with full wheatstone bridge in hand piece. 5' shielded lead with S+,S-,V+,V- plugs into DAQ. Amp only built into DAQ. This is the configuration used by at least one similar product.Pros: Wheatstone bridge is in same environment as sensor. Greatest flexibility of attachment types. Simplest/cheapest DAQ designCons: Attachments are more complex and significantly more expensive. May be unnecessary.As I said, I've probably over-thought this. My original plan was #1. I'm now thinking that #2 would be the better approach. #3 only exists because this is how its done on another product, which dominates this field-I figure they do it for a reason. Thanks in advance.

author: gilbenl
author_rank: Regular Contributor
timestamp: January 20, 2016, 11:52:52 pm
content: Quote from: EmmanuelFaure on January 13, 2016, 01:53:25 amIm using QTI 0.1C resolution thermistors. May ultimately use glass bead thermistors if the application requires. Bridge is composed of Vishay 0.05% 5PPM (PTF10KGCT). Using BB INA126 opamps on the breadboard. May switch to LT1001 since I got a handful for free. ADC is currently LT1867. Total cost of front end stands at just over $100 not including sensors.My biggest issue right now is my coding skills blow and I'm having a heck of a time getting any serial data from the 1867 through my nano. Would definitely appreciate some help in that regard as well.

author: gilbenl
author_rank: Regular Contributor
timestamp: January 21, 2016, 04:13:59 pm
content: Quote from: EmmanuelFaure on January 21, 2016, 12:42:55 amThis is not for a school project. I'm a medical student and am going to be doing neurosurgery research with a new PI who also is into hobby electronics. An AD instruments front end (the market standard) costs about $5000, not including the controller or software. $100 is a pittance. We were hoping to use a plain arduino as a basic DAQ. Quickly discovered this was not the way to go.As I mentioned, my coding is piss poor. Where is the Stdio.h header file coming from? See error below:Arduino: 1.6.7 (Mac OS X), Board: "Arduino Nano, ATmega328"/Users/NLG/Documents/Arduino/libraries/sketch_jan21b/sketch_jan21b.ino: In function 'void loop()':sketch_jan21b:26: error: 'MSB' was not declared in this scopeMSB = SPI.transfer(0);^sketch_jan21b:27: error: 'b0' was not declared in this scopeSerial.print(b0, BIN);^sketch_jan21b:28: error: 'LSB' was not declared in this scopeLSB = SPI.transfer(0);^exit status 1'MSB' was not declared in this scope

author: krapht
author_rank: Contributor
timestamp: January 21, 2016, 08:24:51 pm
content: Cost of thermistor front end parts = $5.Cost of medical liability insurance = $4995

author: gilbenl
author_rank: Regular Contributor
timestamp: January 21, 2016, 08:32:05 pm
content: Quote from: EmmanuelFaure on January 21, 2016, 08:04:13 pmTo be fair, it allows a tremendous number of sensors. However, I've pulled one apart and its effectively opamps and ADCs. Definitely some expertise built into the design, but nothing special.http://www.adinstruments.com/products/powerlabMy project is not intended for thermistors alone. I'm beginning with that because it seems the easiest and I had questions regarding how to configure everything. I'd like to ultimately have temp, pressure, strain etc. AFAIK, all begin with a bridge.That code works, by the way. Thank you. I should be able to to slowly figure out whats going on...at least enough to build the hardware.Also, I should mention none of this is going anywhere near a human. Just in case anyone was freaked out...

author: gilbenl
author_rank: Regular Contributor
timestamp: April 07, 2016, 05:22:54 pm
content: Version 1. Ditched the LT1867 for the simplicity of an LT2983. Could use a second set of eyes to get some proofing/feedback, particularly with regard to the USB (FT232RL) and SD interface as those are parts that I'm less familiar with those. Thanks in advance.

author: daveshah
author_rank: Unknown
timestamp: April 07, 2016, 05:29:23 pm
content: The reset circuit doesn't look right to me - you need a pullup from the switch/capacitor/LTC2983 reset pin to +5V, not a hard connection, otherwise pushing the reset switch will short the power supply, and auto-reset will not work.Also, I believe it is good practice to have pullup resistors (around 47k, not critical) on all of the SD card data lines - on the card side of the HEF4050B - to guarantee that the card starts up in a determinate state.

author: bhalakiya
author_rank: Newbie
timestamp: April 09, 2016, 05:14:35 pm
content: Get HX711

author: gilbenl
author_rank: Regular Contributor
timestamp: April 11, 2016, 04:35:10 pm
content: Thanks for the input and I welcome more. Here's version 2. Note: The battery still says 9V, however, this will be powered using 2X9V in series for 18v. Forgot to change the label prior to upload.Questions:1) Regarding isolation-Are SPI isolators needed between the LT2983 and the micro? Similarly, would isolation of the analog inputs on the front end provide any benefit? Certain sensors will be measuring as low as 5uV changes and there was conversation regarding the potential for ground noise in particular when using mixed sensor types.2) Anyone see any fat that can be trimmed?Changes:1) Added pull up resistors to b/w SD card and buffer for CS, MOSI and CLK2) Corrected orientation of Tx and Rx LEDs on FT2323) Corrected connections of reset circuit, putting resistor/diode after push button.4) Added switchable 10V rail for active sensors5) Replaced through-hole components in power supplies w/SMD