For sensing current of a negative power supply, the designer potential firstly considers a current sensing amplifier since these parts was widely used for current sensing with designed for performance, compact package, low cost and sourcing easy. However, there almost has no current sensing amplifier that can endure common input voltage less than -22V in market for current sensing amplifier.

the difference amplifier INA145 or INA146 was proposed in here for sensing load current for a negative power supply. INA145 and INA146 is comprised by two stage amplifiers. The first stage is a difference amplifier and was used to remove common input voltage but amplify the difference voltage. The second stage is a non-inverter amplifier to amplify the difference voltage again since the difference voltage is quite small. refer figure 4 for details. the common input voltage range for INA145 and INA146 is ±30V and ±100V respectively. This can meet most application for sensing load current for a negative power supply.

Figure 3-4 Current Sensing for Negative Power Supply Based Difference Amplifiers

The recommendation is to not have the designer use a precise amplifier and resistor to build a difference amplifier since the common-mode rejection is related with accuracy of external resistor. This is pretty hard for designer to receive best common-mode rejection by external resistor even 0.1% resistor was used. For INA145 and INA146, best common-mode rejection was received by on-chip precision resistors which are laser-trimmed to achieve accurate gain and high common-mode rejection. Note: To design proper operation of INA145 or INA146, the voltage at the non-inverting input of internal amplifier must be within the linear operating range. The voltage is determined by the simple 1:1(INA145) or 10:1(INA146) voltage divider between pin 3 and pin 1. This voltage must be between V– and (V+) – 1V. Figure 3-4 shows the negative power supply of INA145 was connected to -24V.

Assume sensing resistor is Rs, load current is Is, feedback resistor is R4, input resistor is R3 and output voltage is Vo, Then the output voltage for monitor current as formula 4 shows.