Many systems use current signals to control remote instruments. The advantage of this method is the ability to operate with two remotely connected power supplies even if their grounds are not the same. In these cases, it's necessary for the output to be linear with respect to the input signal, and interference between the grounds must be rejected.
For the circuit in Figure 1, the transfer function is:
IOUT = (VIN /10)/100 Ω IOUT = VIN (V)/1 kΩ
A difference amplifier with a very high common-mode rejection ratio (CMRR), the AD629 is driven by an input signal at pin 3. Its transfer function is: VOUT = VIN, where VOUT is measured between pin 6 and its reference (pin 1 and pin 5). The input (VIN) is measured between pin 3 and pin 2. VCM, the common-mode signal, will be rejected.
In order to reduce the voltage at pin 6, an inverter with a gain of 9 is connected between pin 6 and its reference. The inverter sets the gain of the transmitter in such a way that for a 10-V input, the voltage at pin 6 only changes by 1 V. Yet the difference between pin 6 and its reference is 10 V.
The gain between the noninverting terminal of the OP27 and the output of the AD629 is 1. Therefore, no modulation of the output current will take place as a function of the output voltage VOUT. Resistor R3 is set at 100 Ω to create an input signal scale factor of 1 mA/V.
The OP27 was chosen because, at a noise gain of 10, its bandwidth does not compromise the transmitter.
Shown in Figure 2 is the transfer function of the output voltage (VOUT) versus the input voltage (VIN). Figure 3 demonstrates the excellent ground noise rejection achieved when using this current transmitter.
Reprints
