New gameplay unlocked! Can full differential output be achieved in this way?

As the need for precision continues to increase, fully differential signal chain components are distinguishing themselves by their superior performance, and one of the main benefits of these components is the rejection of noise that can be picked up through signal routing. Since the output picks up this noise, errors often occur at the output and are thus degraded further in the signal chain.

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

One technique is to use an op amp to drive the reference pin, with the positive input being the common mode and the negative input being the center of two matched resistors that connect 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, mismatches 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 may be required on the op amp to limit the overall bandwidth of the circuit. Finally, the gain range of this circuit is determined by the instrumentation amplifier. Therefore, it is not possible to achieve a gain less than 1.

Another technique is to connect two instrumentation amplifiers in parallel with the input switches. This configuration has better matched drive circuits and frequency response than the previous circuit. However, it cannot achieve gains less than 2. This circuit also requires precision matched gain resistors to achieve a purely differential signal. Mismatches in these resistors can cause changes in the output common-mode level, with the same effects as the previous architecture.

Both approaches impose limitations on the achievable gain and the requirements for matching components.

This new circuit provides a fully differential output with precision gain or attenuation using a single gain resistor by cross-connecting the two instrumentation amplifiers as shown in Figure 4. 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 for the non-inverting op amp configuration. Similarly, 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 gain at the output of the buffer is 1.

Gain resistors R3 and R2 set the gain of the circuit, and only one is needed to achieve a fully differential signal. The positive/negative output depends on which resistor is installed. Leaving R3 out will cause the second term in the gain equation to go to zero. This gives a gain of 2 × R1/R2. Leaving R2 out will cause the first term in the gain equation to go to zero. This gives a gain of –2 × R1/R3. Another point to note is that the gain is purely a ratio, so gains less than 1 can be achieved. Keep in mind that using two gain resistors will cause the first stage gain to be higher than the output, since R2 and R3 have opposite effects on the gain. If care is not taken in choosing the resistor values, the result will be an increase in the deviation at the output caused by the first stage op amp.

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

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 to the input signal in amplitude.

As mentioned previously, 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 was chosen to achieve G = 100.

The performance of this circuit is as described by the gain equation. To achieve the best performance, some precautions should be taken when using this circuit. The accuracy and drift of the gain resistors will add to the gain error of the instrumentation amplifier, so choose the appropriate tolerance based on the error requirements.

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

The cross-connection technique maintains the desired characteristics of the instrumentation amplifier while providing additional functionality. Although all of the examples discussed in this article achieve differential outputs, in the cross-connection circuit, the common mode of the output is not affected by resistor pair mismatches, unlike other architectures. Therefore, a true differential output is always achieved. Also, as shown in the gain equation, differential signal attenuation is possible, eliminating the need for a funnel amplifier, which was previously necessary. Finally, the polarity of the output is determined by the location of the gain resistors (using R2 or R3), which adds more flexibility to the user.

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