Instrumentation Amplifier Circuit Question

[vne147]

This is a schematic I got off of eBay from someone selling an "improved" instrumentation amplifier for use with a load cell. Here's a **broken link removed**.

Does anyone have any thoughts about the circuit? Are there any improvements that can be made? I plan on using this in a project with a Xbee. Are there any precautions I should take to avoid interference?

Also, what's the purpose of the AD854 op amp? From what I can see, it's set at unity gain and all it does is provide a reference voltage to the AMP04 IC. Why not wire the VR3 pot directly to the AMP04?

Thanks for any insight you can provide.

 

[ericgibbs]

hi vne,
The REF pin of an IA should 'see' a low impedance, if not the Vref will change as the Vout of the IA changes.
By using a OPA for the Vref, it will be held closely to the voltage set by the pot.

OK.
The Vs +5V will have to be stable as you are using a +V trim on the non inverting input of the IA.

 

[vne147]

Eric,

Thanks for the info. With respect to your comment:

The Vs +5V will have to be stable as you are using a +V trim on the non inverting input of the IA.

I would expect that once the pot is set after initial construction, there would be minimal detrimental affect on V+. Would you agree with that or do you think I should take additional measures to insure stability of the IA input voltage?

 

[Reloadron]

Just to add a tiny bit see page 9 of the PDF as it covers what Eric explained. Additionally the PDF has some more circuits that may be of interest to you.

Ron

 

[vne147]

Just to add a tiny bit see page 9 of the PDF as it covers what Eric explained. Additionally the PDF has some more circuits that may be of interest to you.

Ron

Thanks Ron, I'll check it out.

 

[crutschow]

To insure stability of the amplifier offset you may want to use an adjustable precision reference voltage to generate the pin 5 VREF voltage instead of from the 5V driving an op amp. For example the LTC1258 can be used as an adjustable reference with low output impedance (see PIN FUNCTIONS on bottom of pg. 5 in the data sheet).

 

[vne147]

To insure stability of the amplifier offset you may want to use an adjustable precision reference voltage to generate the pin 5 VREF voltage instead of from the 5V driving an op amp. For example the LTC1258 can be used as an adjustable reference with low output impedance (see PIN FUNCTIONS on bottom of pg. 5 in the data sheet).

Carl,

Thanks for the suggestion. I'll look into that as well.

 

[vne147]

Hello again. I've been working on a design using the circuit we talked about in this thread. Since my design is microcontroller based, I posted another thread in the microcontroller forum asking some additional questions. I just uploaded the schematic to that thread in the microcontroller forum. Here it is:

Instrumentation Amplifier and PIC ADC

Eric is already over there but if everyone else hasn't checked it out yet, I would be grateful it if you could head over there and provide me with some feedback on my design. Even if it only relates to the analog portions, your thoughts are welcome and much appreciated. Thanks in advance!

 

[KeepItSimpleStupid]

I agree that Vref should come from a real reference.

I see no bypass caps for the IC's.

 

[vne147]

I agree that Vref should come from a real reference.

I see no bypass caps for the IC's.

So it sounds like I should nix the AD8541 and replace it with a precision voltage reference IC as suggested by crutschow. I'm still looking into that option and will probably go that direction.

Also, I'll add some bypass caps for the AMP04.

Thanks for the input!

 

[Boncuk]

I suggest to look into the REF200, a completely floating dual current sink.

You might generate your offset correction reference voltages using that.

Boncuk

 

[vne147]

I suggest to look into the REF200, a completely floating dual current sink.

You might generate your offset correction reference voltages using that.

Boncuk

Thanks. I'll check it out.