Differential active probes for oscilloscopes allow measurement of small differential voltages while rejecting common-mode signals. Commercially available differential active probes can be dc- or accoupled. When ac-coupled, however, these probes have a very limited common-mode rejection ratio (CMRR) for low-frequency signals. This is because they use independent filters for each signal line (Fig. 1a).
In a worst-case situation, component tolerances limit the CMRR in the bandpass to:
CMRR (f >> fc) ≈ 20 log f/fc − 20log(tR + tC) + 34 dB
where fc is the corner frequency for the filters, and tR and tC are the respective percent tolerances for resistors and capacitors. If tR = tC = 1%, for example, at f = 10fc, the CMRR can be as low as 54 dB. Furthermore, fc can’t be selected, but it is fixed. The ADA 400A probe from Tektronix, for instance, has fc ≤ 2 Hz (typical). For a gain of 10, at 1 Hz we measured a CMRR of 28 dB when ac-coupled, and 109 dB when dc-coupled.
The differential filter (Fig. 1b) yields a custom corner frequency and a better CMRR. Instead of relying on matched components, the circuit uses a potentiometer to balance the time constant for each filter.
Another circuit, based on a resistor T network, further improves the CMRR (Fig. 1c). In the T network, if R2 were infinite, the CMRR would only be limited by the amplifier, not by the filter. This is because there would not be any way for an input commonmode voltage to result in a differential-mode voltage at the filter output. Nevertheless, R2 must be included in order to provide a bias path for the amplifier inputs. In the ADA 400A probe, it’s possible to disconnect resistor R1 (1 MW) in Figure 1a, but a path to ground for a bias current of 25 pA must always be provided.
The three circuits in Figure 1 were designed using C = 100 nF (tC = 20%), R1 = 2.2 MΩ, R2 = 10 MΩ (tR = 5%) and P = 500 kΩ. Figure 2 shows the connection of the filters to the differential probe. Figure 3 displays the respective CMRR, together with the CMRR for the probe alone, both dc and ac-coupled, when the probe gain was 10. The third circuit provides the larger CMRR improvement at low frequencies. The penalty is a reduced differential input resistance:
Rd = 2R1 + R12/R2 ≈ 5 MW
In addition to their superior CMRR, the circuits in Figures 1b and 1c enable fc to be chosen at will (fc = 1/2πR1C). For example, it’s possible to design an fc larger than powerline frequencies in order to better reject interference.