Differential Opto-Isolated Drivers for Testing Instrumentation Amplifiers

Some of the electrical signals we use are said to "float" relative to the ground. A typical example might be the voltage drop across a shunt resistor in a power supply or a complex biomedical signal such as an electrocardiogram. In this case, an instrumentation amplifier (IA) is used to amplify the differential-mode component of the signal and suppress its common-mode component.

Instrumentation amplifiers need to be tested with real signals during the design process and regularly during actual use. The IA should also be evaluated by applying a known calibrated test signal to its input to determine its accuracy, common-mode signal rejection, and how it is affected by various incorrect connections that may occur during use. A test signal source for medical IA should produce a suitably shaped signal U OUT with an amplitude ranging from a few millivolts and a frequency ranging from zero to several kHz. The source should have (two) differential outputs that can be connected to the corresponding inputs of the IA, as shown in Figure 1.

Figure 1 Differential signal source

The output resistances RG1 and RG2 should be at least a few kilowatts to simulate the characteristics of the objects they will measure in real life. Additionally, both outputs should be galvanically isolated from ground, but should provide a common reference to test the AI's ability to reject common-mode interference.

There are many different types of test signal sources available. Each type, starting with function generators and ending with specialized digital synthesizers, offers different levels of precision and complexity. Many are able to provide signals within the appropriate amplitude and frequency range, and some can even simulate ECG, EEG and other medical signals. However, working with these sources can be challenging because many of them have single-ended outputs and are not sufficiently isolated from ground for common-mode separation testing.

These sources can be tested by adding a driver circuit that converts the single-ended signal to a differential signal and ensures potential separation. This article describes the design, construction, and application of such circuits. Its output may be isolated from ground and provide a "common" signal. Additionally, the impedance of the simulated signal can be adjusted to match the impedance of the single-ended signal source.

Practical optical isolation of analog signals

Isolation between input and output is achieved using an optocoupler (OC), which contains a light-emitting diode (LED) and photodiode (PD) in the same package. The PD acts as a detector, a photoelectric current generator, where the current through the PD is proportional to the light produced by the signal through the LED. 

For applications involving differential signaling, a dual-channel OC with a single LED driving two PDs, such as Vishay's IL300. Dual-channel devices are usually preferred to ensure that any variation between the response of the two channels (due to manufacturing variations) is kept to a minimum. In this application, light from an LED is directed to two PDs, one of which can be used to monitor the amount of light produced by the LED to provide linear feedback for driving the LED. The second PD is used to actually transfer the signal across the isolation barrier to the output. Reference 3 provides several examples of circuits containing OCs. However, all of these examples require the use of an op amp on the output side of the OC, and therefore also require a potential separate (isolated) power supply.

Optocouplers are often used to provide electrical isolation for digital data streams. In these applications, they operate in "saturation mode," in which the LED is driven hard enough to fully saturate the PD when turned on and with little current flow when turned off, resulting in a clean digital pulse train. However, in this application, the OC operates in its linear range, sometimes called photovoltaic mode, where the PD produces a signal proportional to the light from the LED. Our DI uses the photovoltaic mode of the OC to isolate the analog test signal from the signal generator. Figure 2 shows a simple circuit with linear OC where the PD is used in photovoltaic mode, similar to a solar cell.

Figure 2 A simple circuit using a linear optocoupler.

The current through PD1 and PD2 is converted into voltage by load resistors R3 and P1. As long as both voltages (U PD1 and U out) remain within the linear range of PD (less than 50mV in our case), their amplitudes will be proportional to the amount of light produced by the LED. The operational amplifier U1 compares the signal U PD1 with the input signal U IN and drives the LED to make them equal. Trimmer P1 is used to adjust the gain of the circuit (U OUT / U IN), and capacitor C2 prevents oscillation.

The output U OUT (our test signal source) comes from the second photodiode PD2, isolated from ground; its internal resistance is determined by R3. Photovoltaic mode is not typically used with linear OCs because the available output voltage range is limited to a few mV. For this application, the photovoltaic mode is preferred since it does not require any power supply at the output of the OC and the required output signal is anyway small.

Special requirements for isolation changes

The circuit in Figure 2 can only output a positive voltage U OUT (because the current through the LED and the two PDs can only flow in one direction). This problem can be solved by adding a small positive offset to the input signal U IN. Most signal generators provide offset adjustment. However, this also adds a DC offset to the output signal U OUT. If the DC bias output can be tolerated, or the unwanted DC output can be suppressed by adding an RC high-pass filter with an appropriate corner frequency and a modified frequency response is accepted, then the circuit in Figure 2 is sufficient.

If the driver's output signal requires no DC offset, and its frequency response must drop all the way to 0 Hz, the DC offset should be subtracted from the output. In this case, a second battery and a trimmer potentiometer can solve the problem. However, a simpler solution that does not require a second battery is shown in Figure 3. This circuit adds a second DC driven OC (U3) whose output PD is in anti-parallel with the output PD of OC U2. The DC current through OC U3 is set via P7 to compensate for the bias current of OC U2.

Figure 3 Complete schematic of an optically isolated differential driver.

The design also includes a low-power op amp (OPA349), primarily because its input common-mode range exceeds the supply rails by 200 mV and it requires very little power. Therefore, the total current consumption of the circuit is approximately 1 mA. Since the prototype is powered by two AAA batteries, its lifespan should be close to 1,000 hours.

It is important to note that the maximum range of the input signal and the power consumption of the circuit are highly dependent on the bias level. The bias is fixed to 20 mV through resistor divider R5/R6, which sets the bias current through the LED in OC U2 to approximately 500 mA. A similar LED current should be set for OC in U3. In this variation of the original circuit, the input signal does not need to be offset from ground since the resistor divider consists of R4 to R6.

The maximum acceptable input voltage (U in ) for this circuit is approximately ±5 V. Beyond this output, the signal is distorted, partly due to the low bias of 20 mV and partly due to the nonlinear OC range of the PD in U2 at the edge of the photovoltaic mode. For a 1 V pp input signal, a 1 mV pp output signal and harmonics below -40 dB can be expected. Frequency response extends from 0 Hz to approximately 10 kHz (-3dB).

Settings and adjustments

The assembled circuit is shown in Figure 4 below.

Figure 4 Completed circuit. Note that trimmer P1 is omitted since in this case there is no need to calibrate the gain of the circuit.

Adjustment of the circuit begins by applying a sinusoidal signal of approximately 500 Hz and 4 V pp to U IN and observing the input and output (U OUT ) signals using an oscilloscope. NOTE: A 10:1 probe must be used (at least). Then adjust trimmer P1 to obtain an amplitude ratio of 1000:1 on both traces. Finally, trimmer P7 should be adjusted so that the average output signal at U OUT is zero.