DC motor learning (5) current sampling amplifier

In DC motor driving, CSA (current sampling amplifier) ​​is used to collect the current of the motor. The simplest one is the bottom side current sampling. Usually we may not need to use CSA and use ordinary amplifiers to build a differential amplifier. For example, we can Use the analog OPZ302 to build a differential amplifier, as shown in Figure 1 below.

Figure 1: OPZ302 builds a differential amplifier with Gain=100

Generally speaking, there are several points for collecting current, as shown in Figure 2. R6 calls it high-side current sampling, which has a relatively high common mode. R7 calls it inline current sampling, which needs to withstand relatively high dv/dt. R5 calls it bottom-side current sampling, which does not require high common mode. Mode rejection ratio, and does not need to withstand high dv/dt.

Figure 2: Motor current collection point

For the differential amplifier we built with external resistors, since the resistors are not well matched, the common mode rejection ratio is relatively poor and we can only collect the motor current on the bottom side. The short circuit of the load to ground cannot be detected by collecting the bottom side current. If the power supply is connected in parallel with multiple power supplies, it is impossible to detect how much current each power supply draws.

In addition, OPZ302 is a zero-drift op amp, which means that the offset of OPZ302 will not change much when the temperature changes. Maybe the only thing you need to worry about is the temperature drift of the resistor.

In this case, then we just choose a CSA with a relatively high common-mode voltage. We can choose the analog CSA231 with a suitable price and a common-mode voltage as high as 76V, as shown in Figure 3 below.

Figure 3: Analog CSA231

Moreover, the Gain of CSA231 also has multiple levels, which can be applied to various shunt resistors, and there are two packages to choose from. Because the resistor is connected to the power supply, the common-mode voltage will not change significantly. However, when the H-bridge is turned off and the H-bridge enters the braking mode, the high-side current sampling cannot detect the motor current.

So we have to go to the ultimate solution and collect the current of R7. Because the common mode voltage of R7 changes drastically, the usual CSA cannot withstand drastic changes in current. CSA240 has been optimized for drastic changes in common mode. We see how CSA240 handles changes in common mode as shown in Figure 4 below.

Figure 4: PWM common mode rejection by CSA240

The INA240, which was first released by TI, a major American manufacturer, also has PWM rejection. We can take a look at the PWM rejection capability of INA240, as shown in Figure 5. You can see that the OUT of TI's INA240 is smoother, but if you look carefully, you can't bear to look at it. Check the voltage level.

Figure 5: INA240 PWM rejection

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