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Differential amplifiers subtract or condition the difference between two signal inputs, but the name covers circuits built for different jobs. A resistor-based difference amplifier is a straightforward subtractor; an instrumentation amplifier is designed for high-impedance sensor signals; and a fully differential amplifier produces two outputs for signal chains such as differential-input ADCs. Choose by matching the circuit to the source and the next stage—not by treating the three architectures as interchangeable.
What does a differential amplifier do?
A differential amplifier responds to the voltage difference between two inputs. In practical circuits, it also encounters voltage shared by both inputs—the common-mode voltage. How well the circuit rejects that shared component, and how it loads the source, depends on its architecture and implementation.
“Differential amplifier” can mean a basic op-amp subtractor, a discrete transistor differential pair, or an integrated amplifier family. The distinctions below focus on three common approaches to differential signal conditioning: the classical difference amplifier, the instrumentation amplifier, and the fully differential amplifier.
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What are the main types?
Classical difference amplifier
A classical difference amplifier uses an op amp and a resistor network to subtract one input voltage from another, usually producing a single-ended output. It can be a compact, direct solution when the source impedances are low and well controlled and the resistor network can be matched accurately.
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Its limitations matter when measuring real sensors. The inverting input has relatively low input resistance, the two inputs may not present equal resistance, and common-mode rejection depends strongly on resistor matching and balance in the source resistances. Texas Instruments discusses these constraints in its Instrumentation Amplifier application report. A simple subtractor is therefore not automatically a good high-impedance sensor front end.
Instrumentation amplifier
An instrumentation amplifier is intended for differential measurements where the source should be loaded as little as possible and common-mode voltage should be rejected. High input impedance and common-mode rejection make this architecture a standard choice for low-level sensor signals, including strain-gauge bridges, temperature sensors, and pressure transducers. TI’s instrumentation amplifier overview describes the family and its measurement applications.
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Specifications are device-specific. For example, TI lists the INA851 with 22 MHz typical bandwidth at minimum gain, minimum CMRR of 106 dB at gain 10, and minimum CMRR of 120 dB at gains from 100 to 1000. Those figures apply to that device under its specified conditions; they are not generic instrumentation-amplifier performance. See the INA851 product information and check its datasheet for operating conditions and other limits.
Fully differential amplifier
A fully differential amplifier (FDA) has two outputs. The signal of interest is the difference between them, while their shared voltage is the output common-mode. This lets a designer convert a single-ended source to a differential signal, condition an existing differential signal, or drive a differential-input ADC while setting the output common-mode to a compatible value.
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TI’s THS4535 datasheet identifies single-ended-to-differential conversion as a typical use and illustrates an ADC interface using a 1.65 V output common-mode with a 3.3 V ADC. That is an example design value, not a universal ADC requirement. An FDA can also combine gain and active filtering in one stage; TI discusses these functions in its active-filter design brief for differential ADCs.
How do the architectures compare?
| Selection factor | Difference amplifier | Instrumentation amplifier | Fully differential amplifier |
|---|---|---|---|
| Typical job | Subtract two voltages | Condition a differential sensor signal | Condition or drive a differential signal chain |
| Input impedance | Set by resistor network; may be unequal | High input impedance is a key use-case feature | Depends on device and configuration |
| Common-mode rejection | Sensitive to resistor matching and source balance | Often central to sensor measurement | Depends on device and application; verify the datasheet |
| Output | Usually single-ended | Commonly single-ended; some devices provide differential outputs | Differential pair with controllable common-mode |
| Typical application | Simple analog subtraction | Bridge, temperature, pressure, and other sensor inputs | Single-ended conversion, active filtering, and differential ADC drive |
When should you use a fully differential amplifier for an ADC?
Consider an FDA when the ADC has differential inputs and the signal source is single-ended or needs gain, filtering, or common-mode translation before conversion. The FDA’s two outputs can provide the differential drive, and its common-mode control can help meet the ADC’s input requirements. Confirm compatibility from both devices’ specifications rather than assuming that a differential output alone guarantees a suitable interface.
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The ADC input is part of the amplifier’s load. TI’s TIDA-01053 ADC driver reference design highlights the capacitive nature of ADC inputs and the resulting need to consider stability, noise, current drive, and harmonic distortion. Its comparison includes an FDA and a two-amplifier single-ended approach.
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- ADC common-mode and input range: Confirm the FDA can set the required output common-mode and keep both outputs within their allowable swing.
- Differential amplitude and gain: Set the required signal level without exceeding the ADC’s differential input range or the amplifier’s output capability.
- Input loading: Account for the ADC’s input capacitance and any sampling behavior specified for the converter.
- Bandwidth and filtering: Check that the amplifier and any active filter meet the signal bandwidth and settling needs.
- Noise and distortion: Evaluate their effect on the measurement and conversion performance, including SNR, ENOB, and harmonic distortion where relevant.
- Stability and drive: Ensure the FDA remains stable and can drive the actual ADC load; do not rely only on unloaded output specifications.
TI’s comparison of fully differential amplifiers and dual-channel op amps for single-ended-to-differential conversion also identifies distortion, SNR, ENOB, power, common-mode control, active filtering, size, and input impedance as design factors. The best topology depends on the full signal chain, not just the amplifier’s headline bandwidth.
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How to choose the right architecture
- Start with the signal source. For a low-level, high-impedance sensor or bridge, assess an instrumentation amplifier first. For a low-impedance source that needs a straightforward subtraction, a resistor-based difference amplifier may be sufficient.
- Define the destination. If the next stage requires a differential signal—especially a differential-input ADC—evaluate an FDA. If it expects a single-ended signal, verify whether a single-ended output is the simpler fit.
- Check input and common-mode limits. Compare the source’s voltage range and common-mode level with the amplifier’s input range. For ADC drive, also match the ADC’s required common-mode and differential swing.
- Size gain, bandwidth, and noise together. Confirm gain across the needed frequency range, then assess noise and distortion against the measurement or conversion requirements.
- Validate the real load and implementation. Include source resistance, resistor tolerances where applicable, ADC input capacitance, output loading, supply range, output swing, and stability in the circuit review.
These checks prevent a common design mistake: choosing by architecture name alone. A high-CMRR sensor amplifier does not automatically solve ADC output-drive needs, and an FDA’s differential outputs do not make it a universal replacement for a high-input-impedance sensor front end.
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