I was trying to simulate (In LTSPICE) buffer for ADC 08200 that would have 1Meg input impedance it is about to be used with standart oscilloscope probes. But I am having problem with bandwidth limiting. So far I have tried OPA354 and LMH6702 and both suffer from this pitfall.
What is correct way to make high impedance ADC buffer?
So far I have came to this as circuit with best result (as far as bandwidth goes, but it's having very high attenuation). I am simulating 1:10 probe with 9Meg resistor.
//Edit:
I have redesigned the circuit but I am having weird problem with simulation and I think that circuit should work but I am getting very small negative voltage out of OPAMP even thought it's supplied by positive rail only.
This is my schematic now:
Blue trace is output of OPA355 and green trace is non-inverting input.
R11 and R12 are used for offset adjustment. C5 is AC coupling capacitor (if used).
//Edit3:
Fixed model of OPA355 now, but I am having different problem.. on certain frequency it seems that there is some parasitic capacitance charging, but I have no clue where .. Same trace is for non-inverting input, output and inverting input…
Best Answer
An approach to high input impedance buffering with an op-amp is to create a non-inverting unity gain buffer, using a very high input impedance op-amp, such as the Intersil CA3140 (1.5 Tera Ohms), or the Texas Instruments OPA2107 (10 Tera Ohms), both of which have a Gain Bandwidth Product of 4.5 MHz.
In a non-inverting unity gain buffer configuration, the input impedance of the buffer is the input impedance of the op-amp itself, and resistance noise is minimized / none.
One other factor, though, is the input capacitance of these op-amps, 4 pF in the case of both these example op-amps. This capacitance itself would load the incoming signal, if the signal frequency is very high.
As the question does not state the desired bandwidth, the suitability and capacitance impact of the suggested op-amps can not be verified. Using a spice model for one of these in the simulation may help in this.
Based on subsequent comments, for a 100 MHz unity gain bandwidth desired, the Texas Instruments OPA355 or OPA356 would work, with their GBW of 200 MHz, and input impedance of 10 TeraOhms coupled with capacitance of just 1.5 pF.