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Digital Oscilloscope Input Circuitry

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alifred

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I am currently in the final stages of designing a USB Digital Oscilloscope but wondered if anyone knew why quite a few designs use the following circuitry and what its purpose is (image attached).

I have extracted this schematic from the BitScope designs which have been invaluable to the project so far but have also seen it used in PicoScope Digital Oscilloscopes.

I gather the series capacitor (C32) is for AC coupling (or short it out for DC), the 8pF cap is for some protection and D8 and D9 are for clamping but what is the purpose of the resistor voltage divider with capacitors and why is TRM1 variable? I also assume the two 510K resistors add in series to give your ~1M input impedance.

I would be very grateful for any help with working out this circuit!
Fred
 

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C31 does not seem necessary. It's probably to provide some compensation to improve the frequency response, perhaps due to stay circuit inductances.

The voltage divider is apparently to allow for a higher input voltage range than the following circuits can tolerate, or perhaps to also improve frequency response.

Trimming of TRM1 is to give a flat frequency response. Typically you input a swept frequency from a generator and adjust for best response.
 
what is the purpose of the resistor voltage divider with capacitors and why is TRM1 variable?

if i correctly recall my EMC classes, this kind of circuitry ensures stability at high frequency response, the variable capacitor is - i think - used to control the slew rate of that input.

Keep us posted on the advance on that project, seems interesting! :)
 
if i correctly recall my EMC classes, this kind of circuitry ensures stability at high frequency response, the variable capacitor is - i think - used to control the slew rate of that input.
There is no reason to control the slew rate of the input (why would you want to?). And slew rate is normally an active circuit parameter, not a passive circuit one.

The variable cap is for the reason I stated, to adjust the flatness of the frequency response of the oscilloscope. It compensates for the frequency rolloff that would otherwise occur due to the 255kΩ equivalent resistance of the 510kΩ resistors, in series with the amplifier input capacitance.
 
There is no reason to control the slew rate of the input (why would you want to?). And slew rate is normally an active circuit parameter, not a passive circuit one.

The variable cap is for the reason I stated, to adjust the flatness of the frequency response of the oscilloscope. It compensates for the frequency rolloff that would otherwise occur due to the 255kΩ equivalent resistance of the 510kΩ resistors, in series with the amplifier input capacitance.

Agree. you're right, that much more logical :)
 
As previously suggested, R25 and R26 provide an input attenuator and define the resistive component of the scope input impedance as 1M ohm (+/- a few percent).
C72 and TRM1 provide adjustable frequency compensation for the input attenuator.

Oscilloscopes usually have an input impedance of 1Mohm in parallel with 20 or 30pf.
I suggest that C31 is there to bring the input capacitance up into this range.

When divider probes are used, they need to see a scope input input impedance in the 20/30pF range, otherwise the compensation adjustment of the probe will not have sufficient adjustment range to compensate the probe to this scope.

JimB
 
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I agree with JimB.
Also, with a 1:1 probe (or with no probe just a wire) there needs to be some impedance between the real world and the amplifier inside.
With out a 2:1 divider, over voltage would hit the input protection diodes with out current limit and the input amplifier or ADC would likely die.
With the 2:1 divider very little current will hit the protection diodes when the input signal is too big.
 
Thanks for your replies, that seems to make sense.

I have attached a picture of my current spice file and output graph for my current simulations of an input stage. These seem to be acting as expected, except for the reactions in the 10-100MHz range.

Why is the voltage being attenuated here, and what can I do to prevent this, as this is a large part of the bandwidth of my scope.

The 3 traces on the graph are: blue line -> V+ input to U1. Green is the output, and Red is the output from U5.

These opamps have a bandwidth of 350MHz, so shouldn't be being affected.
 

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You have a 10kΩ resistor in series with the input op amps. That will roll off your frequency due to the input capacitance of the op amps. Even 1pF of capacitance will roll off the response at 15MHz. You need a small capacitor across R17 to compensate for that. Re-read my first post.

You can calculate the -3dB rolloff frequency as 1/(2Pi*R*C).

Edit: Just noticed you have a 5kΩ to the virtual ground source so that reduces your equivalent input impedance to 3.3kΩ, but that still will roll off the frequency at a low value, even for small values of input capacitance.
 
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Thanks for all your help - I have now managed to raise the gain to normal over the lower frequencies, now just the higher frequencies left!

Does anyone know how I could 'shift' the drop in magnitude to higher frequencies? I have played around with the values of C2 and R17, however that doesn't alter the frequency range in which the drop occurs.

The traces are as before: Green is the output of U1 and Blue is the output of U5.
 

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Connect the signal generator directly to the input of U1 through C1. If the response is the same, then you know the problem is somewhere in the amp circuit. Perhaps your model for the op amp is not correct.
 
Following your suggestion of looking in isolation, I think there is an issue with the op-amp - or the opamp module in its response to frequencies above 60Mhz, yet the LT1807 has a bandwidth of 350MHz so should be able to cope fine here.

The attached graph shows the input (green) and output (blue) of an single supply non-inverting amplifier using the LT1807. The gain should be 2, but the response across the range 10-100MHz is rather different from this.

How can I make the response more stable across the whole range?
Thanks.
 

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I tried simulatiing your circuit with an LT1807 spice model I downloaded from the Linear Tech website, and also got strange simulation results. It would appear their is a problem with the model.
 
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