I happen to have Rigol DM3058E multimeter that allows measuring both voltage and current of some circuit. This mode has some limitations but it is useful in most (or at least some) cases. As multimeter has dual mode measurement function I quite often wished it could measure (and log) two separate voltages as well. Like, for example, voltages of two batteries in pack while charging. There are measurement racks that can do this and more but such devices are not discussed here.
To work around this I came up with circuit that converts input voltage to current. Current is sourced from separate supply according to input voltage and can be measured by multimeter while doing other voltage measurement. It is, is in a sense, adjustable current source that works best with floating input voltage putting some minimal load on it.
For testing I made prototype using somewhat lower spec TS272C opamps. Is has some limitations but works quite promising. There is additional 5V and 2.5 V converters on prototype board that is not on schematic. As I trust in my DM3058E I hand picked and fine tuned exact resistor values. Initial conversion accuracy is really good. Somewhat problem is that circuit does not work well if separate mains referenced devices are connected to input and output like function generator and multimeter. Although outputs and inputs of these are supposed to be floating in reality they are not. Only thing floating (except for batteries) in my lab is power supply... as it is linear one with true isolating transformer inside. Measuring batteries, however works quite well. Besides 1:1000 range there is 1:200 range settable by jumper but it seems to be slightly off.
I suppose to build the circuit around Texas Instruments TLV9352 opamps and P-channel JFET transistor. TLV9352 opamps are listed surprisingly cheap with wide operation voltage up to 40V and good current sourcing capability.
Specs of the device would be:
Input (measured) voltage up to 40 V that should be matched with power supply for the device. Like, to convert 20V input (referenced to negative rail or differential / floating) 20 V supply should be used.
1:1000 voltage to current ratio. 1V input will source 1 mA to output, 10V will source 10 mA and so on. Probably additional 1:100 range can be used for input voltages up to 4 V. For that some jumpers (not nice) or relays (nice clicking) can be used.
Conversion precision: ±0.5 %. Bandwidth: up to 1 Mhz. Can convert only positive voltages with + input more positive that -.