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- still asymmetrical voltage divider by not switching the input side of the negative dividers
- PCB clearance not suited for 2 kV input voltage
I don't understand your question.Why not single compensated divider strings for positive and negative path?
3 kV/mm is a large multiple of the clearance values suggested by safety and PCB design standards. But you don't even have it. Consider that 2 kV also appears at the open switch contacts after the top divider resistors. Means the switch has to be 2 kV rated.
I mean one multitap voltage divider instead of three.
Parallel capacitances of the compensated dividers are unsuitably high for 100 MHz, even 10 MHz bandwidth.
For one because low C is a figure of merit for a scope probe. You don't want to load the circuit being measured. Have the lowest C possible.
Second there is no reason you can't get the same bandwidth using a multi-tap divider in a simulation where the switches are ideal. And in real life you don't want to pay for many 1meg resistors and a high voltage switch.
For one because low C is a figure of merit for a scope probe. You don't want to load the circuit being measured. Have the lowest C possible.
Second there is no reason you can't get the same bandwidth using a multi-tap divider in a simulation where the switches are ideal. And in real life you don't want to pay for many 1meg resistors and a high voltage switch.
[in+/-]
|
[1meg]
|---------o
[100k]
|---------o o--------Amplifier + or -
[10k]
|---------o
[1k]
|
[GND]
First, the suggestion is one series chain per input (positive and negative)
Second you have different cap values in your sim for the different dividers...decrease the caps and you'll get more bandwidth.
10 pF input capacitance is still large for an active 100 MHz probe but at least acceptable for a number of measurement problems. To realize the bandwidth, the source impedance must be lower than 150 ohms
The latest simulation shows that off-the-shelf E12 capacitors don't allow a precise compensation. Combinations of multiple capacitors and trimmers are usually required.
A real problem arises however if the OP input capacitance is switched between the taps.
Don't use a BNC for a balanced input, it implies the use of screened cable which will inevitably have more capacitance to the outside World than the center core.
The LED/Zener overload indicator probably won't work because it would take enormous voltage to light them through a 20M resistance or higher. By increasing capacitance across the amp inputs they may also reduce the bandwidth. If you need overload indicators, wiring them to indicate excess output from the amp would be more appropriate and you probably need a monostable to make them light for long enough to see transient voltages.
Use two screened cables, ground the shields and use the inners one to each channel. in your present configuration, not only is there a risk of imbalance as mentioned before but the shield is actually half the input so it actually shields nothing anyway.So how would I determine when to use a screened cable, or capacitance over noise?
Use two screened cables, ground the shields and use the inners one to each channel. in your present configuration, not only is there a risk of imbalance as mentioned before but the shield is actually half the input so it actually shields nothing anyway.
If you can afford the slight bandwidth trade-off, move the first resistor in the voltage divider to the probe tip so it isolates the capacitance of the cable from the source.
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