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What is a differential amplifier?
A differential amplifier has two inputs, and its output is proportional to the difference between their voltages. Let the inputs be V1 and V2. Using the convention that the output rises with V2 − V1, define:
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- Differential input: Vd = V2 − V1, the signal difference the amplifier is meant to detect.
- Common-mode input: Vcm = (V1 + V2)/2, the average voltage shared by the two inputs.
The ideal transfer relation is Vout = Ad(V2 − V1), where Ad is differential gain. There is no common-mode term: changing both input voltages by the same amount leaves the output unchanged. Reversing which input is designated positive reverses the sign convention. Analog Devices describes the general behavior as amplifying the input difference while rejecting voltage common to both inputs (Differential Amplifier).
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How does common-mode rejection work?
Suppose a desired signal produces a small difference between the two input leads, while interference couples onto both leads in nearly the same way. The amplifier ideally responds to the difference and suppresses the shared component. This is why differential signaling can help in environments where noise is picked up similarly by both conductors.
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- This product is a differential to single ended amplifier, with a 270MHz bandwidth and a very high common mode rejection ratio (CMRR) at high frequencies.
- This product can be effectively used as a high-speed instrument amplifier or to convert differential signals into single ended signals.
- AD8130 has excellent common mode suppression（ 70dB@10MHz ）, allowing the use of low-cost, non shielded twisted pair cables without fear of being damaged by external noise sources or crosstalk.
- The AD8130 has a wide power supply voltage range of +5V to ± 12V, which can withstand a wide range of common mode and differential mode voltages while maintaining signal integrity.
- Size: 43mm × 38mm； Power supply voltage range: ± 5V~± 12V
Common-mode rejection ratio (CMRR) describes differential gain relative to common-mode gain. A higher CMRR means less output response to a common-mode input compared with the response to a differential input. Rejection is not the same as elimination of every unwanted signal: noise that differs between the leads appears as differential input and cannot be rejected as common-mode noise. Nor does high CMRR make an out-of-range input safe or valid.
Low-amplitude ECG sensing is one example of the application class: Analog Devices describes amplifying a signal on the order of a few millivolts before digitization (Differential Amplifier). That example does not mean a basic difference-amplifier circuit by itself meets medical-device safety or performance requirements.
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- Bandwidth: AD620/623 is lower than 50KHz at high gain and greater than 1MHz at low gain Input signal type: single-ended signal or differential signal
- The maximum magnification: more than 2000 times (actual measurement), due to the gain assembly resulting in greater fluctuations in the waveform band, it is recommended to use high-gain multi-stage amplification
- Saturation output amplitude: AC peak 10Vpp (±5Vpp)
- This module can be enlarged 1000 times, the circuit layout is regular, MINI, design layout and wiring are reasonable
What does the ideal op-amp assumption mean?
In simplified ideal-op-amp analysis, the amplifier has infinite open-loop gain. With negative feedback and while the circuit remains in its linear operating range, feedback drives the voltage difference between the op amp’s inverting and non-inverting inputs toward zero. This is often called a virtual short: the terminal voltages are treated as nearly equal for analysis, but the terminals are not physically connected.
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This result depends on negative feedback and linear operation; it is not a universal property of every amplifier circuit. An op amp’s output is bounded by its supply rails, and its inputs and output must stay within the device’s operating limits. If the output saturates or an input is outside its permitted range, the ideal feedback analysis no longer describes the circuit’s behavior (Analog Devices University Program: Chapter 2).
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- Input Voltage: DC 3-12V, Signal Input Voltage: 100uV-300mV, Module Size: 32 X 22mm/1.26 X 0.87inch(L*W)
How does the four-resistor difference amplifier work?
A common implementation uses one op amp and four resistors. In the standard arrangement, one input feeds the inverting branch through R1 with feedback resistor R2; the other input feeds the non-inverting branch through R3, with R4 connecting that branch to ground. For this labeling, when the resistor ratios match—R2/R1 = R4/R3—the ideal output is:
Vout = (R2/R1)(V2 − V1)
Here, V1 is applied to the inverting branch and V2 to the non-inverting branch. If all four resistors have equal values, the circuit is a unity-gain subtractor; other matched ratios provide differential gain. The equation is not a guarantee for arbitrary resistor values: the ratio match is essential for common-mode cancellation. Analog Devices explains the topology and its resistor-ratio requirement in A Deeper Look into Difference Amplifiers.
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- Low noise, low distortion, with good signal transmission capability.
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- High CMMR(common-mode rejection ratio).
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Why do resistor matching and real-world limits matter?
Ratio mismatch converts common-mode voltage into error
If the resistor ratios do not match, the two input paths no longer cancel common-mode voltage completely. Some of that shared voltage then appears at the output as an error that can be mistaken for a real differential signal. The issue is the relationship between resistor values, not merely whether each resistor is close to its nominal value; a matched precision resistor network can help control ratio error and drift.
For illustration, Analog Devices calculates that with unity gain and a perfect op amp, 1% resistors limit CMRR to 50 V/V, or about 34 dB; with 0.1% resistors, the illustrative value is 500 V/V, or about 54 dB. These are calculations under those stated assumptions, not guaranteed specifications for every four-resistor circuit (A Deeper Look into Difference Amplifiers).
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The inputs must be within common-mode range
CMRR describes rejection within the amplifier’s applicable operating conditions. The common-mode voltage—the average of the input voltages—must still fall within the selected amplifier’s permitted input common-mode range, and the output must have enough swing to represent the amplified difference. A circuit can fail to behave as intended even if the differential signal is small, when the shared input voltage is outside those limits.
CMRR and bandwidth are related
CMRR can decrease as frequency increases. A rejection figure at low frequency therefore may not describe performance at the frequency of the actual signal or interference. Check CMRR over the relevant frequency range rather than treating a single value as constant (Choosing a Precision Amplifier Topology).
How should you choose a differential-amplifier topology?
A four-resistor op-amp difference amplifier is one option, not the automatic choice for every differential signal. Compare the needs of the signal and source with the limits of the candidate circuit or amplifier:
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- Common-mode conditions: Determine the expected common-mode voltage and check the allowed input common-mode range.
- Rejection at frequency: Compare CMRR at the signal and interference frequencies, not just a low-frequency figure.
- Sources and loading: Consider source impedance and whether the inputs load the source unequally.
- Matching and drift: Account for resistor-ratio tolerance and changes with temperature.
- Headroom and bandwidth: Check supply requirements, output swing and bandwidth against the full signal range.
Topology selection depends on signal type, source impedance, required CMRR and gain accuracy; the appropriate choice also depends on the application’s input range and frequency needs (Choosing a Precision Amplifier Topology). An operational amplifier IC is the active element in the standard four-resistor implementation, but no particular part can be selected responsibly without those application requirements.
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