Fully Differential Outputs Using Single-Ended Instrumentation Amplifiers

Additionally, differential signals can achieve twice the signal range of single-ended signals on the same power supply. Therefore, a fully differential signal has a higher signal-to-noise ratio (SNR). The classic three-op-amp instrumentation amplifier has many advantages, including common-mode signal rejection, high input impedance, and precise (adjustable) gain; however, it falls short when a fully differential output signal is required. Several methods have been used to implement fully differential instrumentation amplifiers using standard components. However, they have their own shortcomings.

One technique is to use an op amp to drive the reference pin, with the positive input being common mode and the negative input being the center of two matched resistors connecting the outputs together. This configuration uses the instrumentation amplifier output as the positive output and the op amp output as the negative output. Since the two outputs are different amplifiers, a mismatch in dynamic performance between these amplifiers can greatly affect the overall performance of the circuit.

Additionally, the matching of the two resistors causes the output common mode to move with the output signal, which can result in distortion. When designing this circuit, stability must be considered when selecting the amplifier, and a feedback capacitor on the op amp may be required to limit the overall bandwidth of the circuit. Finally, the gain range of this circuit depends on the instrumentation amplifier. Therefore, it is impossible to achieve a gain less than 1.

Another technique is to connect two instrumentation amplifiers in parallel with the input switch. This configuration has a better matched drive circuit and frequency response than the previous circuit. But it cannot achieve a gain less than 2. This circuit also requires precisely matched gain resistors to achieve a purely differential signal. A mismatch in these resistors results in a change in the output common-mode level, with the same effect as in the previous architecture.

Both approaches have limitations on achievable gains and matching component requirements.

By cross-connecting two instrumentation amplifiers, as shown in Figure 4, this new circuit uses a single gain resistor to provide a fully differential output with precision gain or attenuation. By connecting the two reference pins together, the user can adjust the output common mode as desired.

The gain of In_A is derived from the following equation. Since the input voltage appears at the positive terminal of the input buffer of instrumentation amplifier 2 and the voltage at the other end of resistors R2 and R3 is 0 V, the gain of these buffers follows the equation that applies to the non-inverting op amp configuration. Likewise, for the input buffer of instrumentation amplifier 1, the gain follows the inverting op amp configuration. Since all resistors in the differential amplifier are matched, the buffer output has a gain of 1.

Gain resistors R3 and R2 set the gain of the circuit, and only one resistor is needed to achieve a fully differential signal. Positive/negative output depends on installed resistor. Not installing R3 will cause the second term in the gain equation to go to zero. From this, the gain is 2 × R1/R2. Not installing R2 causes the first term in the gain equation to become zero. From this, the gain is –2 × R1/R3. Another point to note is that gain is purely a ratio, so gains less than 1 can be achieved. Remember, since R2 and R3 have opposite effects on gain, using two gain resistors will make the gain of the first stage higher than the output. If you are not careful when selecting the resistor values, the result will be an increase in the deviation at the output due to the first stage op amp.

To demonstrate this circuit in action, we connected two AD8221 instrumentation amplifiers. The data sheet lists R1 as 24.7kΩ, so when R2 is 49.4kΩ, a gain equal to 1 is achieved.

CH1 is the input signal of In_A, CH2 is VOUT_A, and CH3 is VOUT_B. Outputs A and B are matched and inverted, and the difference is equal in amplitude to the input signal.

As mentioned earlier, one limitation of other techniques is the inability to achieve attenuation. According to the gain equation, using R2 = 98.8kΩ, the circuit will attenuate the input signal by a factor of two.

Finally, to demonstrate high gain, R2 = 494Ω is chosen to achieve G = 100.

The circuit behaves as described by the gain equation. For best performance, some precautions should be taken when using this circuit. The accuracy and drift of the gain resistor will increase the gain error of the instrumentation amplifier, so choose the appropriate tolerance based on the error requirements.

Since capacitance on the Rg pin of the instrumentation amplifier can result in poor frequency performance, attention should be paid to these nodes if high frequency performance is required. Additionally, a temperature mismatch between the two instrumentation amplifiers can cause system offset due to offset drift, so layout and loading care should be taken here. Using a dual-channel instrumentation amplifier, such as the AD8222, can help overcome these potential problems.

Cross-connect technology maintains the desired characteristics of an instrumentation amplifier while providing additional functionality. Although all of the examples discussed in this article implement differential outputs, in cross-connect circuits the common mode of the outputs is not affected by resistive mismatch, unlike other architectures. Therefore, a true differential output is always achieved. Furthermore, differential signal attenuation is possible as shown in the gain equation, which eliminates the need for a funnel amplifier, which was previously necessary. Finally, the polarity of the output is determined by the position of the gain resistor (using R2 or R3), which adds more flexibility to the user.