Design Article
Test op-amps for input bias current
David R. Baum and Daryl Hiser, Texas Instruments
2/7/2012 3:36 PM EST
In part 1 of this series, "Circuits test key op amp parameters," we addressed some of the basic op amp (operational amplifier) tests such as VOS (offset voltage), CMRR (common-mode rejection ratio), PSSR (power-supply rejection ratio), and Aol (amplifier open-loop gain). In this article, we discuss two methods for testing input bias current. The method we choose depends on the magnitude of the bias current. We'll present the sources of errors that you should consider when testing the devices. The next installment of this series will cover a configurable test circuit that lets you make all of the measurements described in this installment.
Data sheets often provide a table of bias currents for an op amp's noninverting and inverting inputs, iB+ and iB-, respectively. The difference between these two inputs is the input offset current, IOS. On the bench, you might be tempted to test the positive input bias current by using the circuit in Figure 1a because the amplifier in this configuration is stable, which will work.
Follow the jump directly to Test&Measurement World to read the entire article.
Data sheets often provide a table of bias currents for an op amp's noninverting and inverting inputs, iB+ and iB-, respectively. The difference between these two inputs is the input offset current, IOS. On the bench, you might be tempted to test the positive input bias current by using the circuit in Figure 1a because the amplifier in this configuration is stable, which will work.
Follow the jump directly to Test&Measurement World to read the entire article.
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hm
2/9/2012 4:21 AM EST
Very informative application note. Is it possible to get it in PDF format from TI web site?
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skrib01
2/17/2012 3:13 PM EST
Why would you use an electrometer? Are you measuring fA or sub pA? If you are you will need additional guarding and shielding to eliminate noise. The problem arises when placing a capacitive node on the input to an op amp with out a tiny series resistor may drive your op-amp into oscillation....we've made iB measurements with this technique before...
On a different note the test circuit should have capacitance in parallel with the input resistors, and the loop amp. This is a tricky servo to compensate.... It may look easy in simulation but the rubber hits the road in the lab.
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drbaum
2/28/2012 11:28 AM EST
We used an electrometer to test opamps like the OPA128, which is 75fA typical. It required special guarding, like you mention, and everything had to be super clean on the test board and socket.
Compensation issues will be discussed in a latter article. We suggest always monitoring any test circuit with a scope since compensation for Ib measurements can be difficult.
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