Electronic – Programmable differential amplifier

• The "resistors in the voltage divider" presumably refer to Resistor 6 and Resistor 7 in the schematic, providing the biased signal voltage to one of the inputs of the op-amp.
  • These dividers would be selected such that their combined resistance in parallel is significantly smaller than the input impedance of the op-amp. Otherwise, the current into the op-amp input will change the bias voltage - like putting an additional resistor of comparable value in parallel with one leg of the divider.
  • Also, they should be of high enough value that the current through them does not draw an unreasonable amount of current from DC Voltage Source 1.
  • Finally, the resistors should be of sufficient power rating to tolerate the power dissipation P = I^2 x R for the current drawn through them, I = V(DC1) / (R1 + R2)
• The op-amp gain resistors would depend on the gain desired. Since the schematic does not indicate inverting and non-inverting inputs, it is not clear (at least to me) whether this is intended as an inverting or non-inverting amplifier, and what the intention of that part of the design is.
• Dynamic voltage dividers can be implemented in a variety of ways, which include using a digital potentiometer IC, an resistor array with switched legs, etc.
  • If one assumes that the question envisages only 3 possible input signal voltages, a dynamic voltage divider may not be needed. One could structure 3 voltage dividers, and switch the appropriate one in using a very small R_ds(on) MOSFET, such that the MOSFET's resistance does not discernibly impact circuit behavior.
  • Just 2 voltage dividers would suffice, if the intention is merely to provide a counter-bias of -1 Volts for the 0-2 Volt signal when that is the source.
• Overvoltage protection (presumably referring to the input signal voltage) can be achieved by using a voltage limiting zener diode circuit, at the simplest level.
  • Depending on the currents involved and the behavior desired, other solutions such as voltage foldback or SCR-based crowbar circuits may be called for - apparently not applicable in this case.

Given the spread of voltages and frequencies that you need to measure I would firstly concentrate on the 100Hz signal - say you attenuate it by 30 - this would take it from 72Vp-p to 2.4Vp-p and to keep the impedances low I would use a capacitive divider formed by a 10pF capacitor and a 290pf capacitor.

Given also that you maybe be interested in the dc levels (you will be for the 5MHz signal) I would add resistors across the 10pF and 290pF that have the same ratio of impedance say 1Mohm across the 10pF and 34.5kohm across the 290pF.

Next is the problem of the 5MHz signal and you need to attenuate this by (say) 2:1 - this means the 34k5 and 290pF need to be replaced with (say) another set of 10pF // 1Mohm. I would then consider that these extra components were permanently in-circuit and that the 290pF and 34k5 were in fact 280pF // 35k7 so that when in parallel with 10pF // 1Mohm gave 290pF // 34k5.

I would consider use a low capacitance JFET for the switching: -

Also shown is the OPA354 which is a low input capacitance, unity-gain-to-40MHz op-amp with decent offset voltage, bias currents and noise. It is typically run from a split supply of +/-2.5V centred on 0V. The supply is split like this to accommodate the 100Hz AC signal but it gives a tiny problem for the 5MHz signal because, with the attenuation of 2:1 at 5MHz the input would equal the power rail - this means the attenuation should be designed to be a tad lower at maybe 2.5:1. These values can be easily calculated from the ones shown i.e. the lower 1Mohm becomes 667k and the lower 10pF becomes 15pF. These changes will have a small knock-on effect on the 35k7 and 280pF.

I would also consider the 15pF cap becoming a trimmer to get the peaking just right - setup would be just like on an oscilloscope probe when trimming. JFET - 2N3819 should work ok - it has a drain capacitance of about 1pF.

Why take this approach? To work effectively across the voltage and frequency range needed if only resistors were used, it is likely that input capacitance on op-amps and switching circuits (like JFETs or analogue switches) would make a bit of a mess of things. With "same ratio" capacitors these effects are reduced. More to follow as I think a little more about this.