Instrument amplifierInstrumentation amplifiers typically consist of DA output stages with buffered inputs.

The first advantage over DA is the ability to easily change the differential gain (Adm) of the device using an external resistor (Rg). Secondly, the input of the current sensor is connected to the in-phase input of the buffer amplifier. The in-phase input has high impedance, which means that there is almost no load on the system bus voltage, allowing for the detection of smaller system currents. However, one drawback is that the input common mode voltage of IA is limited by its power supply voltage. Therefore, IA is usually used for low-end measurements (Figure 7).

IA can be used in high-end measurements (see Figure 8). Designers must ensure that the system bus voltage is within the input common mode range of IA (determined by its power supply voltage).

current shunt monitor

A shunt monitor is a device that has almost no load on the system and allows induction of current under high common mode voltage conditions. This feature can be achieved by designing a unique input level. Figure 9 depicts a typical operational amplifier common emitter input stage, while Figure 10 illustrates the common base input stage of CSM.

To illustrate this benefit, let's take a look at an example of a high-end measurement (Vcm=70V) for DA and CSM.

One possible CSM solution includes INA282, whose input common mode range extends from -16V to 80V. At Vcm=70V, the input bias current is approximately 25 µ A. One possible DA solution includes INA146. INA146 can accept input common mode voltage of ± 100V under ± 15V power supply. The common mode input impedance of the device is 55k Ω. Therefore, the device will draw 70V/55k Ω=1.27mA from the bus power supply. This comparison indicates that under high common mode voltage, CSM can produce higher accuracy (or a larger range of load currents).

As shown in the example, DA has a larger common mode voltage range compared to CSM.

Finally, in order to maintain a good common mode rejection ratio and ideal price, CSM usually has fixed returns. Exceptions include current output devices and digital devices, where the current output devices of current sensors require external precision resistors to set the gain.

directional

The solutions for bidirectional load current include most DA, IA, and CSM. A data table typically indicates whether a device can be used in a bidirectional system. A key indicator of bidirectionality is the availability of reference inputs, as shown in the DA and IA diagrams in Figures 3 and 6, respectively. As described in Part 1 of this series, placing a voltage on the reference pin of the device will reference the output to that voltage.

Input/output voltage range

Attention must be paid to ensuring that the input and output voltage ranges of all devices mentioned in this article are within the specifications of the data sheet. The scope is listed in the "Recommended Operating Conditions" table of its data table. At least violating the input range of the device can ensure non-linear operation. Depending on the duration and severity, exposing the input of the device to voltages outside the recommended range in the data manual can also damage the device. The output range of the device directly depends on the power supply voltage. It is necessary to ensure that the product of input voltage and device/circuit gain is within the output range specifications of the device.

This is particularly important when selecting power supply voltage for IA solutions. As shown in Figure 6, the traditional three operational amplifiers IA usually have a graph in the data table showing the relationship between the output voltage range and the input common mode range under different power supply conditions.