I did some basic tests on current measurement functions.
Some thoughts on the design -
- high current relay K703 offloads the 1A and 10A ranges to shunt R702 (Vishay VCS301 0.1R 4W resistor) as soon as possible, so subsequent relays and copper traces can be smaller/thinner.
- the shunt/TIA select relay K709 unloads the capacitance of the shunt chain/switches from the TIA input, when the TIA is active.
- the high-current relays K702,K703 are latching RT424 and were tested for leakage, at least to 1pA level.
- BOOT2 used to bootstrap the protection is switchable depending on whether shunts or TIA are active.
The shunts drop 100mV.
So amplifier gain = 100x, except for 10A where gain =10x.
Excessive self-heating on the R702 for 10A may be an issue.
So 3A or even 1A (P=100mW) could be a practical limit, if the VCS301 doesn't have a heatsink.
The current shunts get switched using relays, while the sense taps are switched using analog-switch U703.
The spare relay pole could also be used to switch the sense tap.
But the analog-switch has an advantage as a precaution against extra Seebeck/thermocouple effects due to relay construction.
The 100x gain, means thermocouple offsets get amplified along with the small burden voltages.
On this point, relay K709 (shunt/TIA select) is shown placed in the shunt chain, after R702 where DCI-HI is sensed.
I think it should move before R702, to eliminate relay thermocouple effects, even if that would require changing to a high-current relay.
The TIA 10uA,1uA, 100n ranges are stable with feedback caps of 22p.
Output range is +-10V.
A TIA has a large inherent dynamic-range. but range-switching is added using several feedback resistors.
This should reduce the Vos drift present in the measurement signal, compared with a fixed value TIA and switchable gain amplifier.
I am not sure if a chopper-stabilized op would work for ACI ranges (possible feature development), due to intermodulation distortion.
I have a preference to avoid zero-drift amps due to inexperience and high apparent complexity (high Ios/input offset currents, switch noise, emi source, one-time complexity).
Perhaps the TIA should be a compound amplifier, to help equalize BW across ranges, similar to the main amplifier.
But I am not sure if this applies to a TIA where current-gain is the important point, at least on the op-amp input side.
Also, bandwidth for different ranges, can be somewhat controlled for - with the value of C in the Rinput/Cf ( 10k/22p=723kHz ).
The input protection has not been tested. A single bridge-rectifier repackage (gbj1508) would be simpler than discrete dpak bjts and smb diodes.
It would probably be better to demonstrate that the more complicated circuit with BC tied bjts is needed before using it.
I am not sure if some inductance should defined on the input near the input fuse.
Or whether the inductance shown in some schematics is added as a workaround for small dimensioned fuses blowing on transient currents.