Reduce Amplifier Drift when load is near input impedance

In summary, the conversation discusses the use of the AD625 instrumentation amplifier with a 1 G-Ohm input impedance. The output drift is negligible when the load has a resistance of 100 kOhm, but becomes significant and unacceptable with a load resistance of 100 MOhm. The individual is looking for post hoc tricks to reduce the output drift, and suggestions include using a higher z opamp or implementing guarding techniques. The conversation also mentions the use of an electrometer amplifier or the ADA4530-1 for better results. Finally, the conversation advises paying close attention to shielding, cabling, and grounding to avoid ground loops and minimize stray field and common-mode pickup.
  • #1
Hyo X
101
11
I built a voltage amplifier based on the AD625 instrumentation amplifier. This chip has a 1 G-Ohm input impedance.

When the load has a resistance of 100 kOhm, output drift is negligible.
When the load has a resistance of 100 MOhm, output drift is significant and not acceptable.

Are there any post hoc tricks to limit the drift of the output? tricks with ground loops or something? thanks
 
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  • #2
Why would you want such a high load resistance?
 
  • #3
Thats the lowest resistance of the samples I characterize. It is set by physical limits and we can't reduce it. Other samples have 10 Gohm or higher resistance.
 
  • #4
Are you confusing source and load impedance? The load is what the amplifier drives.
 
  • #5
marcusl said:
Are you confusing source and load impedance? The load is what the amplifier drives.
I must have my terminology wrong!
The sample resistance must be the source. it is the resistance between the V+ and V- inputs of the amplifier. The load resistance should be negligible as this output voltage goes to a data acquisition system.

How can i reduce the drift of the output of the voltage amplifier when the source resistance is close to the input impedance of the amplfier?
 
  • #6
I assume you meant "source impedance"

a higher z opamp might help
http://www.ti.com/lit/ds/symlink/opa128.pdf

a common trick is "guarding" the input pins so as to minimize leakage into the summing junction
http://www.ti.com/lit/an/snoa664/snoa664.pdf
upload_2016-3-9_13-5-46.png
 
  • #7
Without knowing your application in detail, it is difficult to say where this "drift" is coming from--possibly out of the air, given your high source impedance. Instrumentation amplifiers have extensive features to eliminate stray field pickup and common-mode pickup, but they must be implemented properly. Be sure to read Analog Devices's application notes carefully, not just the data sheet. AN-244 will get you started. Shield your source in a Faraday shield; pay close attention to cabling and connection topologies, cable shields, how grounds are routed, and proper power supply practice to avoid ground loops there. That's the best one can offer without sitting in your lab.
 
Last edited:
  • #8
You should consider using an “Electrometer Amplifier” as a unity gain buffer before the AD625 IA.
EAs have an input bias current measured in fA. Consider also the ADA4530-1

As suggested by Jim Hardy, the output should be used to guard the input. The shield or screen on any input cable used should also be driven as a guard. It is often a benefit to lift the input pin from the PCB to eliminate surface contamination currents.
 
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1. What is amplifier drift and why is it a concern when the load is near the input impedance?

Amplifier drift is the tendency of an amplifier to deviate from its original output value over time. This can be caused by various factors such as temperature changes, aging of components, and input voltage variations. When the load is near the input impedance, the amplifier can experience a higher level of sensitivity to these factors, leading to a greater amount of drift.

2. How does the input impedance affect amplifier drift?

The input impedance of an amplifier refers to the impedance seen at the input terminals of the amplifier. When the load is near the input impedance, the amplifier has to work harder to maintain a stable output, as the load and input impedance are in close proximity. This increased workload can contribute to a higher level of amplifier drift.

3. What are some common techniques for reducing amplifier drift when the load is near the input impedance?

There are several techniques that can be used to reduce amplifier drift in this scenario. These include using precision components, implementing temperature compensation, and using feedback circuits to stabilize the amplifier's output. Additionally, proper layout and shielding can also help minimize drift.

4. Can the type of amplifier affect its susceptibility to drift when the load is near the input impedance?

Yes, the type of amplifier can play a role in its susceptibility to drift. For example, op-amps with high input impedance are more susceptible to drift compared to ones with lower input impedance. Differential amplifiers are also less prone to drift compared to single-ended amplifiers since they can reject common-mode signals that can contribute to drift.

5. Are there any trade-offs when implementing techniques to reduce amplifier drift?

Yes, there can be trade-offs when trying to reduce amplifier drift. For instance, using precision components and temperature compensation techniques can increase the cost of the amplifier. Feedback circuits can also introduce noise and reduce the bandwidth of the amplifier. It is important to carefully consider these trade-offs and choose the best approach for the specific application.

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