How to accurately inject offset voltage in active probes?

[dmderev]

Could anyone share experience/suggestions how to inject offset voltage in something like 1:5 or 1:10 active probes?
Specifically, if I have a calibrated programmable voltage (-1V...+1V), and I have a probe with 1:10 front end divider, how do I make this offset apply to the front stage (eg. FET) amplifier in a way, that does not depend on device under test impedance, or whether the probe head is connected or not?

One option is to apply the voltage to the bottom end of the divider's low side resistor. But in this case the voltage applied to the amplifier will change with the probe's tip connection.

Some active probes chose the dual-amp architecture, where the DC goes through a low pass DC amplifier, and AC goes through a FET buffer path. In this case, the offset is applied to the DC channel's summing node. But dual-amp architecture requires perfect match of the frequency responses to avoid "bump" at the crossover point.

Any ideas how to do accurate offset injection in a single amp, wideband probe?

[David Hess]

Could anyone share experience/suggestions how to inject offset voltage in something like 1:5 or 1:10 active probes?
Specifically, if I have a calibrated programmable voltage (-1V...+1V), and I have a probe with 1:10 front end divider, how do I make this offset apply to the front stage (eg. FET) amplifier in a way, that does not depend on device under test impedance, or whether the probe head is connected or not?

There are three ways I have seen used.

The oldest is simply to apply the ground referenced voltage to one side of a differential input amplifier.  The old Tektronix 7A13 and earlier differential comparators do it this way.

Another way is to apply the reference voltage to the reference input of an instrumentation or difference amplifier.

A more recent way is to convert a differential input voltage to a current, and then provide current feedback from the output.  Then the offset "voltage" can be applied as a current to the summing point, usually through a reference input like with an instrumentation amplifier.  This is more suitable for a high frequency design.  Analog Devices and Linear Technology have complete ICs which perform this function.
 

One option is to apply the voltage to the bottom end of the divider's low side resistor. But in this case the voltage applied to the amplifier will change with the probe's tip connection.

Passive offset probes do exactly that.  Some active probes also do this with the offset voltage applied through the input divider.