# Total impedance of scope probe

I would like to measure the total impedance of the scope + 20:1 attenuator + x1 probe

EDIT: Circuit modified according to Bruce Abbott's recommendation

My scope's input impedance is : 1 MΩ ±1%, in parallel with 12 to 13 pF (Picoscope 3204B) The x1 probe is a picotech TA375 with input resistance equals to that of the scope and input capacitance of 57 pF + Cscope = 69 to 70 pF The 20:1 attenuator is a Hantek HT-201 with an input impedance of 1.053 MΩ. But I don't know how the voltage divider with this resistance value could achieve 1/20 voltage attenuation? Also the Hantek website doesn't specify the input capacitance for this attenuator!

• Where did your schematic come from? Commented May 8, 2022 at 6:20
• @BruceAbbott, the basic schematic comes from the web but I modified the position of the attenuator. Commented May 8, 2022 at 8:16
• @BruceAbbott, I updated the schematic according to your explanation. Commented May 8, 2022 at 10:27

## 2 Answers

The 20:1 attenuator is a Hantek HT-201 with an input impedance of 1.053 MΩ. But I don't know how the voltage divider with this resistance value could achieve 1/20 voltage attenuation?

The circuit below has an input resistance of 1.053 MΩ and attenuation of 20:1 with better than 1% accuracy (1.007 V out for 20 V in). Therefore I conclude that these are probably the values used.

simulate this circuit – Schematic created using CircuitLab

My scope's input impedance is : 1 MΩ ±1%, in parallel with 12 to 13 pF (Picoscope 3204B) The x1 probe is a picotech TA375 with input resistance equals to that of the scope and input capacitance of 57 pF + Cscope = 69 to 70 pF

Assuming R1 and R2 etc. have negligible capacitance, Cp should be Cs / (20-1) = 0.63 ~ 0.68 pF. According to this test, the parasitic capacitance of a 1206 size SMD resistor is ~0.05 pF, so Cp should be ~0.57 ~ 0.63 pF. However the exact value required for a flat bandwidth is very critical. The attenuator doesn't appear to have any compensation adjuster, so it may have larger fixed capacitors placed in parallel with R1 and R2 to 'swamp out' parasitic capacitances - perhaps 10 pF across R1 and 180 pF across R2. This would make the attenuator input capacitance ~9.5 pF, for a probe input capacitance of ~67 pF.

This is fairly easy to design and simulate attenuation and input impedance from a current source. The key is that it's not a 10M scope probe but designed for fuel injectors so impedance and voltage ratios are non-standard.

Here's a rough simulation.