ICT resistance testing
Brian Kerridge -- Test & Measurement World, 9/1/2002 2:00:00 AM
Some in-circuit testers (ICTs) provide a signal connection, often called an "Ohms guard," to suppress the effects of parallel resistance and capacitance, which lets the ICT use a much faster read rate for high-resistance measurements. In practice, you make this guard connection to a selection of nodes in the region of the unknown resistor, generally by trial and error.
The figure shows the circuit configuration that ICTs use to measure resistance. A current sink, switched in decades typically from 100 nA to 10 mA, draws current from the op amp's output through the unknown resistance Rx, which is in the feedback path of that op amp. For example, when Rx is 1 kΩ and the current sink setting is 1 mA, then a digital voltmeter (DVM) reads 1 V from which the ICT can calculate that Rx = 1 kΩ.
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Consider the same Rx, now with an Rp1/Rp2 parallel path. If Rp1 and Rp2 are 500 Ω, then the DVM reading would become 0.5 V, which the ICT would read as 500 Ω—a 50% error. By introducing the guard connection, the DVM returns 1 V, which corresponds to the correct value, 1 kΩ. The key to this circuit is that the op amp actively maintains Lo and Rx at the same signal reference. The guard is merely a direct hardware connection to that reference. Ideally, there is zero voltage across Rp2, so it becomes invisible to the circuit. The more nodes you can locate and connect to guard, the more accurately you can measure Rx. The circuit's limitation comes when the combined effects of all Rp1s in parallel finally overload the op amp's output drive.
The combined effects of all Rp2s depend on the precision of the op amp and its input offset voltage (Vio). For example, if Vio is ±1 mV, and if all the Rp2s in parallel equal 10 Ω, then ±10 µA flows through Rx—an error of 1% in measuring 10 kΩ.
Reading speed becomes a problem when an ICT measures high resistances, because adjacent capacitance extends settling time. If Rx= 10 MΩ, then 0.01 µF across Rx takes 0.1 s to charge, which limits the speed to 10 readings/s.
With a guard connection, the maximum current drive from the op amp's output now charges Cp1. Assuming a 10-mA drive current and a capacitance of 0.1 µF, Cp1 now charges in just 10 µs, which effectively removes the read-rate limit. You'll obtain similar speed benefits by connecting the guard to the screen of your connecting cables to reduce the effects of cable capacitance.
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