How can you design a low-power, cost-effective amplifier circuit that converts a differential input into a single-ended output? Many applications require low-power, high-performance differential amplifiers to process small differential signals and convert them into ground-referenced outputs. These circuits typically deal with large common-mode voltages on both inputs. The differential amplifier effectively rejects this common-mode voltage, leaving only the differential signal to be amplified and output as a single-ended voltage.
The common-mode voltage can be either AC or DC, often larger than the actual differential signal. However, the ability of the amplifier to suppress this voltage decreases at higher frequencies. Amplifiers in the same package offer better matching, similar parasitic capacitance, and eliminate the need for external wiring. As a result, dual-channel amplifiers with high bandwidth tend to perform better than discrete solutions in terms of frequency response.
A straightforward approach is to use a dual-channel precision amplifier with a resistive gain network, as shown in Figure 1. This configuration allows for a simple and effective way to convert a differential input into a single-ended output with adjustable gain. The overall system gain can be calculated using Equation 1:
$$ \text{Gain} = \frac{R_F}{1\,k\Omega} $$
Where $ (V_{IN1} - V_{IN2}) $ represents the differential input voltage.
This method provides greater stability in the presence of EMI or RFI, making it ideal for environments with noise issues. It’s especially beneficial when measuring small signals from devices like thermocouples, strain gauges, or bridge-type pressure sensors, which often operate in noisy conditions.
The circuit not only measures the voltage difference between the two sensor terminals but also offers common-mode rejection, enhancing performance over traditional single-ended configurations. Additionally, the sensor's ground can differ from the analog ground, which is crucial in many applications where precise reference points are needed.
System accuracy depends heavily on the tolerance of the resistors used in the network. The circuit can adjust its gain by setting the ratio of $ R_F $ to $ R_{G1} $, assuming $ R_{G2} = R_{G1} $ and amplifier B has a gain of -1.
For instance, the ADA4807-2 dual amplifier, operating at 180 MHz, can be configured as an inverting amplifier for this application, offering lower noise performance. It also features a low quiescent current (1 mA per amplifier), making it suitable for low-power, high-resolution data acquisition systems.
The input common-mode voltage can exceed the supply voltage, so rail-to-rail output capability is essential in cases of large common-mode signals or high output voltages. For example, if an ADC expects a 0 V to 5 V single-ended input, but the signal source is a differential voltage from a sensor bridge, this circuit helps translate the signal properly.
Figure 2 illustrates the performance of the differential-to-single-ended amplifier, showing how the system gain changes with different resistor values. With a 1 V peak-to-peak differential input at 1 kHz, gains of 1, 2, and 4 are demonstrated.
This circuit is particularly useful for measuring small differences between two large voltages. Consider a battery-powered system using a 3 V power supply to monitor a Wheatstone bridge with 1% resistor accuracy. The circuit will reject any common-mode noise and amplify the attenuated bridge signal according to the set gain. If driving an ADC, some level shifting may be required to ensure the output stays within the 0 V to 5 V range.
Overall, this design offers excellent distortion performance and low power consumption. Using dual op-amps reduces system costs while improving overall performance, making it a smart choice for many applications.
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