It is said that some of the electrical signals we use “float” relative to the ground. A typical example may be the voltage drop on the shunt resistance in the power supply or complex biomedical signals, such as ECG. In this case, the instrument amplifier (IA) is used to amplify the differential mode component of the signal and suppress its common mode component.

The instrument amplifier needs to be tested with real signal in the design process and regularly in actual use. The IA should also be evaluated by applying known calibration test signals to its inputs to determine its accuracy, common mode signal suppression, and how it is affected by various incorrect connections that may occur during use. The test signal source used for medical ia shall generate a properly shaped signalU OUT, the amplitude range is several millivolts and the frequency range is 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 1Differential signal source

The output resistors Rg1 and Rg2 should be at least several kilowatts to simulate the characteristics of the objects they will measure in real life. In addition, both outputs shall be electrically isolated from earth, but a common reference shall be provided to test AI’s ability to suppress common mode interference.

There are many different types of test signal sources available. Each type, starting with a function generator and ending with a dedicated digital synthesizer, provides different levels of accuracy and complexity. Many can provide signals in appropriate amplitude and frequency range, and some can even simulate ECG, EEG and other medical signals. However, using 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 test IA by adding a driver circuit that converts single ended signals into differential signals and ensures potential separation. This paper introduces the design, structure and application of this circuit. Its output may be isolated from ground and provided with“public”Signal. In addition, the impedance of the simulation 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 a photodiode (PD) in the same package. The PD acts as a detector, i.e. a photoelectric current generator, in which the current passing through the PD is proportional to the light generated by the signal passing through the LED.

For applications involving differential signals, 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 changes between the responses of the two channels (due to manufacturing changes) are kept to a minimum. In this application, light from the LED is directed to two PDS, one of which can be used to monitor the amount of light generated by the led to provide linear feedback for driving the LED. The second PD is used to actually transmit the signal across the isolation barrier to the output. Reference 3 provides several circuit examples including OC. However, all these examples require the use of an operational amplifier on the output side of the OC, so a potential separate (isolated) power supply is also required.

Optocouplers are commonly used to provide electrical isolation for digital data streams. In these applications, they operate in a “saturation mode”, in which the driving force of the LED is sufficient to fully saturate the PD when turned on and there is little current when turned off, resulting in a clean digital pulse sequence. However, in this application, OC operates in its linear range, sometimes referred to as photovoltaic mode, where PD generates a signal proportional to the light from the LED. Our Di uses OC’s photovoltaic mode to isolate the analog test signal of the signal generator.Figure 2A simple circuit with linear OC is shown, in which PD is used in photovoltaic mode, similar to solar cells.

Figure 2A 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 example), their amplitude will be directly proportional to the amount of light generated by the LED. The operational amplifier U1 sends the signalU PD1And input signalCompare u inAnd drive the LEDs to make them equal. The trimmer P1 is used to adjust the gain (u out / u in) of the circuit, and the capacitor C2 prevents oscillation.

The output u out (our test signal source) comes from the second photodiode PD2, which is isolated from the ground; The internal resistance is determined by R3. Photovoltaic mode is not usually used with linear OC because the available output voltage range is limited to a few MV. For this application, photovoltaic mode is preferred because it does not need to provide any power at the output of OC, and the required output signal is very small anyway.

Isolation changes for special requirements

The circuit in Figure 2 can only output positive voltage u out (because the current through the LED and 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 bias to the output signal u out. If the DC bias output can be tolerated, or the unwanted DC output can be suppressed and the modified frequency response can be accepted by adding an RC high pass filter with appropriate corner frequency, the circuit in Fig. 2 is sufficient.

If the output signal of the driver needs no DC bias,alsoIf the frequency response must drop to 0 Hz, the DC offset shall be subtracted from the output. In this case, a second battery and a trim potentiometer can be used to solve the problem. However,In Figure 3Shows a simpler solution that does not require a second battery. A second DC driven OC (U3) is added to the circuit, and its output PD is antiparallel with the output PD of OC U2. The DC current through OC U3 is set through P7 to compensate for the bias current of OC U2.

Figure 3Complete schematic diagram of optical isolation differential driver.

The design also includes a Low-Power Operational Amplifier (opa349), mainly because its input common mode range is 200 mV beyond the power rail, and it requires little power. Therefore, the total current consumption of the circuit is about 1 mA. Since the prototype is powered by two AAA batteries, its service life should be close to 1000 hours.

It should be noted that the maximum range of the input signal and the power consumption of the circuit largely depend on the bias level. The bias is fixed to 20 mV by the resistive voltage divider R5 / R6 to set the bias current through the LED in OC U2 to approximately 500 mA. A similar LED current shall be set for OC in U3. In this variant of the original circuit, since the resistive divider consists of R4 to R6, the input signal does not need to be offset from ground.

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

Setting and adjustment

The assembled circuit is as followsFigure 4As shown in.

Figure 4Completed circuit. Note that the spinner P1 is omitted because in this case, it is not necessary to calibrate the gain of the circuit.

The adjustment of the circuit starts with applying a sinusoidal signal of about 500 Hz and 4 V PP to u in and observing the input and output (u out) signals with an oscilloscope. Note: a 10:1 probe must be used (at least). Then adjust the trimmer P1 to get the 1000:1 amplitude ratio on the two tracks. Finally, the spinner P7 should be adjusted so that the average output signal at u out is zero.

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