Joey, the datasheet on the ADS1242/43 recommends using EMI filters (figure 8, pg 21) for the load cell's i/o.
What range values would you recommend for these sort of LC filters?

 

hey sorry I didn't read the whole thread. I just post #1 a few weeks ago. Forgive me if this is not applicable, but I just got this in my inbox from sparkfun. The lowest cost all-inclusive load cell amplifier I have come across. Maybe you will have some use for it.
https://www.sparkfun.com/products/1...ail&utm_term=0_fa5287abaf-f508e94455-62251285

 

hey sorry I didn't read the whole thread. I just post #1 a few weeks ago. Forgive me if this is not applicable, but I just got this in my inbox from sparkfun. The lowest cost all-inclusive load cell amplifier I have come across. Maybe you will have some use for it.
https://www.sparkfun.com/products/1...ail&utm_term=0_fa5287abaf-f508e94455-62251285

Very interesting chip, this HX711.

There are two things that I liked:

• On-chip power supply regulator eliminates the need for an external supply regulator to provide analog power for the ADC and the sensor
• There is no programming needed for the internal registers. All controls to the HX711 are through the pins.

Programming all the registers for a 24bit ADC is normally a lot of work! This simplifies things a lot.

There are a few things that worry me, though. Like offset drift and noise. It doesn't seem that the chip is using AC excitation to the bridge. But I guess the only way to find out if it's good enough is to get one and test it. And at $10 dlls it's very much worth the try.

Thanks!

 

Programming all the registers for a 24bit ADC is normally a lot of work! This simplifies things a lot.

<sarc>
Yes. It is much easier to vary the number of clock pulses during readout after each conversion...
</sarc>

 

Programming all the registers for a 24bit ADC is normally a lot of work!

If you want a 24-bit sigma-delta ADC that doesn't require a lot of configuration, try this one:

http://www.linear.com/product/LTC2440

 

If you want a 24-bit sigma-delta ADC that doesn't require a lot of configuration, try this one:

http://www.linear.com/product/LTC2440

Very, very nice... and it's reasonably priced too! And from what I'm seeing, it can also be used and synched with AC excitation.

 

I've been working with load cells for a long time now, and one of the problems that I have not yet been able to solve to my satisfaction is offset drift. Offset drift is the drift in output voltage over time even when the cell is under no load.
A while back, I found this very interesting circuit that excites the cell using a 400Hz AC signal:


View attachment 86446

Strain Gage Measurement Using An AC Excitation
​

Strain gage measurements are often plagued by offset drift, 1/f noise, and line noise. One solution is to use an ac signal to excite the bridge, as shown in Figure 6-14. The AD8221 gains the signal and an AD630AR synchronously demodulates the waveform. What results is a dc output proportional to the strain on the bridge. The output signal is devoid of all dc errors associated with the in-amp and the detector, including offset and offset drift.
In Figure 6-14, a 400 Hz signal excites the bridge. The signal at the AD8221’s input is an ac voltage. Similarly, the signal at the input of the AD630 is ac; the signal is dc at the end of the low-pass filter following the AD630.
The 400 Hz ac signal is rectified and then averaged; dc errors are converted in an ac signal and removed by the AD630. Ultimately, a precision dc signal is obtained.
The AD8221 is well-suited for this application because its high CMRR over frequency ensures that the signal of interest, which appears as a small difference voltage riding on a large sinusoidal common-mode voltage, is gained and the common-mode signal is rejected. In typical instrumentation amplifiers, CMRR falls off at about 200 Hz. In contrast, the AD8221 continues to reject common-mode signals beyond 10 kHz.
If an ac source is not available, a commutating voltage may be constructed using switches. The AD8221’s high CMRR over frequency rejects high frequency harmonics from a commutating voltage source.

I have several questions regarding this circuit:

• Would the AC signal have to be precise and with zero-drift too? (I'm assuming that's a no)
• Is there any other demodulator chip that could be used? The AD630 costs more than 20 bucks!
• How precise would the ±15V sources have to be? Would simple 7815 and 7915 regulators do?
• Is there a different circuit you may suggest that I should try, so as to accomplish my zero-drift goal?

Thanks everyone for your input

"Chopper" circuits have long been used to stabilize DC instrumentation. This seems like an elegant way to do it.

Eric

 

http://www.analog.com/media/en/training-seminars/tutorials/MT-055.pdf
Here's a nice tutorial

 

Very, very nice... and it's reasonably priced too! And from what I'm seeing, it can also be used and synched with AC excitation.

If it were me, I wouldn't bother with AC excitation; I doubt it would yield any substantial improvement over the LTC2440's offset, offset drift, and gain drift specs, and unless done very skillfully could actually degrade performance instead of improving it. With parts like the LTC2440, and the chopper-stabilized opamps available these days, why bother?

Just my opinion...

 

And for a chopper-stabilized, auto-zeroing opamp, check this one out:

http://www.maximintegrated.com/en/products/analog/amplifiers/MAX44246.html

 

If it were me, I wouldn't bother with AC excitation; I doubt it would yield any substantial improvement over the LTC2440's offset, offset drift, and gain drift specs, and unless done very skillfully could actually degrade performance instead of improving it. With parts like the LTC2440, and the chopper-stabilized opamps available these days, why bother?

Just my opinion...

I'm already half-done with the AC circuit anyway... I'm going to see where this takes me, and then maybe I'll go back and try the LTC2440 with a chopper-stabilized opamp.

 

And for a chopper-stabilized, auto-zeroing opamp, check this one out:

http://www.maximintegrated.com/en/products/analog/amplifiers/MAX44246.html

Impressive... I can see from the diagram that they configured three opamps to work as an instrumentation amplifier.
Question: Are there chopper-stabilized, auto-zeroing instrumentation amplifier chips out there?

 

Are there chopper-stabilized, auto-zeroing instrumentation amplifier chips out there?

Google and ye shall find. Here's some light reading matter for you:

http://www.ti.com/lit/ds/symlink/pga280.pdf
http://pdfserv.maximintegrated.com/en/an/AN4437.pdf
http://datasheets.maximintegrated.com/en/ds/MAX4460-MAX4462.pdf
http://datasheets.maximintegrated.com/en/ds/MAX4208-MAX4209.pdf
http://www.ece.gatech.edu/academic/courses/ece6414/S09/Projects/Team4_Draft1_ECE6414_S09.pdf
http://www.eecs.umich.edu/courses/eecs522/w11/project/group3report.pdf

 

Google and ye shall find. Here's some light reading matter for you:

http://www.ti.com/lit/ds/symlink/pga280.pdf
http://pdfserv.maximintegrated.com/en/an/AN4437.pdf
http://datasheets.maximintegrated.com/en/ds/MAX4460-MAX4462.pdf
http://datasheets.maximintegrated.com/en/ds/MAX4208-MAX4209.pdf
http://www.ece.gatech.edu/academic/courses/ece6414/S09/Projects/Team4_Draft1_ECE6414_S09.pdf
http://www.eecs.umich.edu/courses/eecs522/w11/project/group3report.pdf

Ok... I get it... you've almost made a convert out of me... I might be a little stubborn, but I'm definitely not obstinate.

So these marvelous chips will help me avoid the dreaded offset drifts... but what about thermocouple drifts at junction points in the circuit? Wouldn't AC excitation be the only answer to that?

 

So these marvelous chips will help me avoid the dreaded offset drifts... but what about thermocouple drifts at junction points in the circuit? Wouldn't AC excitation be the only answer to that?

I would imagine so; but keep in mind that AC excitation will do nothing to diminish errors occurring within the strain gauge bridge itself, whether due to temperature, aging, hysteresis, or whatever. I may be wrong, but my gut sense is that those errors will prove larger than the DC errors you've been concerned with. I'm not very familiar with what can be achieved with AC excitation these days and I haven't been paying very close attention here, but I have my doubts about whether it can perform miracles.

 

I would imagine so; but keep in mind that AC excitation will do nothing to diminish errors occurring within the strain gauge bridge itself, whether due to temperature, aging, hysteresis, or whatever. I may be wrong, but my gut sense is that those errors will prove larger than the DC errors you've been concerned with. I'm not very familiar with what can be achieved with AC excitation these days and I haven't been paying very close attention here, but I have my doubts about whether it can perform miracles.

Agreed. In fact, I've asked @cmartinez to prove that AC excitation will perform better than a good d/s running DC. He hasn't taken up the challenge.

 

... I have my doubts about whether it can perform miracles.

I agree... but I'm only looking for improvements here... and a not too-expensive (or excruciating) learning experience...
I am most definitely going to follow your advice about the auto-zero opamps, after I'm done with this.

Can you do me a favor? Can you check this circuit? I want to clip voltages delivered from an instrumentation amplifier and going into an ADC, to make sure that those voltages never exceed what's tolerated by the ADC.
I'm going to sim the circuit anyway, but I'd still like your opinion as to whether including this circuit in my design would affect its performance or not.

Thanks in advance!

 

Agreed. In fact, I've asked @cmartinez to prove that AC excitation will perform better than a good d/s running DC. He hasn't taken up the challenge.

well... I ain't no expert sling-wielding rock-throwing biblical David who's up to that challenge, for sure... all I can do is respectfully question what you guys tell me in the hopes that I end up understanding what I'm being told...
Until now, the only source of knowledge I've had on this subject has been derived from reading many datasheets and application notes over the years... that is, reading those documents unguided by knowledgeable people that know better.
I sincerely thank you all for your altruistic efforts in helping me here.

 

Can you do me a favor? Can you check this circuit?

EEEEEEAAAAAAGHGHGHGH!!!!!!!!

Where did you find that circuit, anyway-- chiseled amongst the hieroglyphs on the wall of some Egyptian pharaoh's tomb????? That circuit is ANCIENT.

If you want to limit the input to your ADC (say, to between ground and Vdd), I recommend simply putting a small resistor (a couple of hundred ohms should do) between the instrumentation amp output and your ADC input, plus a pair of clamp diodes on the ADC input, one to ground and the other to Vdd.

 

EEEEEEAAAAAAGHGHGHGH!!!!!!!!

Where did you find that circuit, anyway-- chiseled amongst the hieroglyphs on the wall of some Egyptian pharaoh's tomb????? That circuit is ANCIENT.

If you want to limit the input to your ADC (say, to between ground and Vdd), I recommend simply putting a small resistor (a couple of hundred ohms should do) between the instrumentation amp output and your ADC input, plus a pair of clamp diodes on the ADC input, one to ground and the other to Vdd.

AVERT YOUR EYES!!!!

Thanks for the laugh... I've been having a s***tty day and I needed the comic relief.

I've already tried simulating what you've just told me (which is also what a couple of other people here have suggested too), using the resistor and the diodes to clip the voltage. But that arrangement will only clip voltage higher than Vdd (or lower than 0V) by the diode's forward voltage, which in a typical schottky diode is about 450mv. In my case I'd like to clip the voltage at exactly Vdd and 0V, respectively, if possible. I'm all eyes and ears if you have a better idea.

To answer your question, I found that circuit in the last pages of the attached file... the relics of which where found being firmly grasped by the still-juicy, but thoroughly mummified remains (if that were possible) of the evil priest Imhotep.