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I am designing a load cell amplifier. The load cell output/amplifier input is 2mv/v, and the amplifier's output is 3V. I need an A/D's sampling speed to be 200k-500k.
Would the INA128 be OK? if not please recommend a better amplifier and A/D. Currently I'm using a PGA308 for the amplifier, and the MSP430F5659's built in 12Bit A/D for the A/D, but the repeatability is too low, varying by +/-3-4 LSB. I believe a more precise A/D, such as a 16bit one, will contribute to better repeatability, as well as a faster amplifier.
Shawn,
Sorry for all the questions, but I need a little more info to fully help you out.
Best regards, Art
Art, thank you for getting back to me.
1. This is a closed loop feedback system, where we are continuously sampling the signal from a force load cell, and adjusting the motor to increase or decrease the force in real time. We need to sample at minimum 200k, and are currently averaging two samples for each point. We probably won't need to go over 500K bandwidth.
2. Right now, I'm using the A/D built into the MSP430F5659 microcontroller. It's 12 bits, so I'm wondering if changing to a standalone A/D with a higher precision would improve the repeatability.
3. Right now, I'm using 5V for the PGA308. I can increase it up to +/-15V if necessary.
4. You're right, the gain needs to be at least 500, so the INA128's bandwidth is too low.
5 & 6. Thanks for the design tips. I'll check them out.
7. The easiest and preferred solution is to just tweak the existing PGA308 design. I have been calibrating it, but I can't seem to get the error down to 0.1%. Right now, it's around 0.3 - 0.4%. I am wondering if a tweak to the design will solve the repeatability problem. I attached my PGA308 design, and how it feeds into the microcontroller's A/D. Would it be possible to get the accuracy to be better than 0.05%?
Thank you!
Hi Shawn,
Just to jump in on Art's comments, the ADC12 found in the MSP430F5659 has total unadjusted error of ~ 4-5 bits. Can you increase your sample average to 4 or even 8? That should improve the results as well.
Tom,
Thanks for that extra detail.
Shawn,
That is a lot of information! I hope it helps. Once you decide on a path you would like to peruse I can help you with the details.
Best regards,
Art Kay
Art,
Thanks for all the information! Regarding point 7, I added the RC on the A/D because there were a lot of noise coming from elsewhere. Now I need to figure out how to filter that out without it affecting the bandwidth.
I'm testing all the options now and will definitely get back in touch with you.
-Shawn
Art,
Sorry, is the PGA309_calibration_procedure pdf part of another document? It makes reference to tables which aren't in the document, and I don't see the table numbers in the PGA309 user's guide. Anyplace I can get the document? Thank you!
-Shawn
Art,
Sorry, is the PGA309_calibration_procedure pdf part of another document? It makes reference to tables which aren't in the document, and I don't see the table numbers in the PGA309 user's guide. Anyplace I can get the document? Thank you!
-Shawn
Shawn,
I hope this helps clarify the difference between PGA308 and PGA309. Also, I hope you understand the role of the OTP in PGA308 and look-up table on PGA309. Based on some of your previous comments, I think that you may be close to your bandwidth goal with the PGA308. You may want to look at the system bandwidth after you find gain and offset to make sure it meets your needs.
Best regards,
Art