Hello,
I need a way to measure low current on a negative voltage -24V.
I there a way to connect a current sensing amplifier for this negative common mode.
The parts I found are limited to -22V so will not work.
Thanks
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Hey Appreciated Engineer,
One way to use a current sense amplifier (CSA) (that cannot sense on a -24V rail) is to use a Zener regulator that creates an acceptable floating voltage on top of the -24V source. There are other discrete circuits to create regulated floating voltages.
Once you you determine how to bias the amplifier, you then need to determine how to level shift the negative output voltage into a ground level that is acceptable for an ADC that is reference to 0V. There are various options to do this such as isolated amplifier, isolated ADCs, hall-effect current sensor, discrete analog level shifting circuits, or by using a digital CSA (power monitor) and then use a digital isolator to level-shift the communication bus.
It seems you may only need an analog amplifier, and you also want to measure small currents. The best CSA to measure small currents is capacitive-coupled, high-input impedance amplifiers (INA186, INA190, INA191, INA2191). These amplifiers have 2.3MOhm to 4.6MOhm input impedance, low quiescent current, ENABLE pins, low offset, and good precision.
I have provided an examples below. I used the INA2191 because the other devices do not currently work with floating supply voltages.
Many CSAs cannot accept higher than 5V single-ended supply voltages, so you would need a Zener diode with a max reverse breakdown of 5V. Then you would choose a Zener current limiting resistor (R1). In the circuits below I_R1 = (Vz-Vbus)/R1. You have to choose R1 carefully so it pulls enough current to keep Zener stable, but not too much current that it burns the resistor or drains you -24V supply. Thus, this method may exceed power limits you have.
The only way to mitigate the Zener power loss is to use a Zener with a larger reverse breakdown, another type of voltage regulator circuit, or use a resistor and high decoupling capacitance as shown in the bottom right circuit. Below on the right shows the latter option, where I simply use a resistor divider to create a floating 5V source and the provide strong decoupling capacitance to help minimize loading, which is the reason we usually do not recommend this method; however, the INA2191 (and its associated family) have low quiescent currents so the effects of loading the VS pin is less compared to other higher power amplifiers.
The circuit on the left is simply showing one option to level shift the output voltage to a ground level for a ground level ADC.
The floating supply voltage could also applied to any operational amplifier or instrumentation amplifier as well provided the amplifier can work with Vcm close to ground (the voltage at its V- or Vee pin).
Here are some other circuits which use the same principle.
Sincerely,
Peter