Differential Amp Input Resistors: Matching & CMRR

This is not a textbook derivation. It’s a decision guide for engineers and buyers who need a differential amplifier input resistor network that holds CMRR, stability, and repeatability in production: ratio matching, TCR tracking, noise tradeoffs, layout symmetry, and an RFQ-ready checklist.

Jump to what you need

Answer Box Search intent What it is CMRR & matching Tolerance & TCR Discrete vs network Noise & bias Layout symmetry Troubleshooting RFQ checklist Recommended models FAQ

Precision resistor network and op-amp on PCB highlighting common input resistors and CMRR

One-Screen Answer (Selection + Procurement)

In a differential amplifier, the op-amp is rarely the CMRR limiter. The common input resistors (and their matching to the feedback resistors) typically set your real CMRR ceiling. If the resistor ratios aren’t matched, or if their TCR tracking and thermal symmetry are poor, common-mode noise turns into differential error — and the design becomes “mysteriously noisy” in production.

Pick the right input resistors if…

You specify ratio matching (not only tolerance).
You require TCR tracking across temperature.
You control layout symmetry and thermal gradients.
You choose resistor values that balance noise, bias offset, and loading.

Common buyer mistake

Buying “0.1% resistors” and assuming high CMRR. Tolerance is not ratio matching, and discrete parts rarely track temperature the same way. If you need stable CMRR, specify ratio tolerance and TCR tracking — or use a matched resistor network.

Decision shortcut

If you target ≥80 dB CMRR across temperature, strongly prefer a matched resistor network (ratio & TCR tracking specified). If you’re building cost-sensitive designs with modest CMRR needs, discrete resistors can work — but only with symmetry, controlled sourcing, and realistic expectations.

Search Intent: What People Really Need

Queries like “differential amplifier common input resistor” are rarely academic. They’re almost always about selection, validation, or troubleshooting: “What values should I choose?”, “Why is my CMRR low?”, “Why does it drift with temperature?”, or “Should I use a resistor network?”. This page keeps every technical point tied to a design or procurement decision.

Oscilloscope showing common-mode input producing unintended differential output due to resistor mismatch

What “Common Input Resistors” Mean in a Differential Amplifier

In the classic 4-resistor differential amplifier, the two input resistors (often one on each input leg) are sometimes called “common input resistors” because they set the front-end impedance and participate in the resistor ratio relationships that determine CMRR.

Why procurement should care

In production, you can buy the “same value” resistors from multiple sources and still lose performance. What matters is not only value, but ratio matching, TCR tracking, and mechanical/thermal symmetry. These must be specified, not assumed.

CMRR Reality: Resistor Ratio Matching Sets the Ceiling

Ideal CMRR assumes perfect resistor ratios. In real hardware, small ratio errors convert common-mode signal into differential error. That means: a “perfect” op-amp cannot fix a mismatched resistor network.

Rule of thumb (design planning)

~60 dB CMRR typically needs ~0.1% ratio matching.
~80 dB CMRR typically needs ~0.01% ratio matching.
~100 dB CMRR generally needs ~0.001% ratio matching + excellent thermal tracking.

Procurement impact: if you only specify “0.1% tolerance” you are not actually specifying ratio matching.

What goes wrong in sourcing

Discrete resistors with the same tolerance are not guaranteed to track each other. Different lots, vendors, packages, and placement temperatures create ratio drift. That shows up as “CMRR is fine on the bench but fails in the field.”

Tolerance vs TCR Tracking: Why “0.1%” Can Still Drift

Tolerance describes initial value. For CMRR stability, you also need TCR tracking (how well resistors move together with temperature). Even if two resistors start matched, different TCR causes ratio drift, which reduces CMRR across temperature.

Procurement language that actually works

Ratio tolerance (e.g., 0.01% max) for the paired ratios.
TCR tracking (e.g., 2–5 ppm/°C tracking) for the ratio pair.
Long-term drift and stress sensitivity if this is a precision measurement design.

Comparison of discrete resistors versus matched resistor network emphasizing ratio matching.

Discrete Resistors vs Matched Resistor Networks

Discrete resistors

Pros: flexible values, easy sourcing, low cost.
Cons: weaker thermal tracking, assembly stress variation, more CMRR scatter.
Best for: moderate CMRR designs, cost-focused builds, wide tolerances.

Matched resistor networks

Pros: ratio matching + thermal tracking on a shared substrate.
Cons: fewer value choices, footprint constraints, sometimes higher cost.
Best for: high CMRR, precision sensing, wide temperature range.

Decision rule

If you cannot afford rework, calibration headaches, or field drift — use a matched network. If you can tolerate broader performance scatter and have strong layout control, discrete parts can work.

Resistor Value Selection: Noise, Bias Offset, and Loading

The “best” value depends on your source impedance and measurement bandwidth. Higher resistances increase thermal noise and bias-current induced offset. Lower resistances reduce noise and bias sensitivity but load the source more.

When lower values win

Low-level sensing (mV signals)
High bandwidth measurement
Inputs sensitive to bias-current offset

When higher values help

High source impedance sensors
Battery-powered low-current systems
When input loading must be minimized

Procurement note

If you change resistor values late to “fix noise,” you may also change bias offset, stability, and EMC susceptibility. Treat values as a selection decision with system impact — not a last-minute patch.

PCB thermal gradient near one resistor showing how temperature differences cause ratio drift.

Layout Symmetry: Where “Perfect” Networks Still Fail

CMRR is not only electrical — it’s also physical. Unequal trace lengths, different copper areas, proximity to hot components, and asymmetrical routing create temperature gradients and parasitics that turn common-mode signals into differential error.

Layout rules that actually matter

Place resistor pairs close together; prefer mirrored geometry.
Keep both input legs equally exposed to heat sources and airflow.
Avoid routing one input leg near switching nodes or high dV/dt nets.
Keep parasitics symmetrical: trace length, via count, copper pour shape.

Troubleshooting Matrix: Fast Path to Root Cause

Symptom	Likely cause	Fix / selection action
CMRR much lower than expected	Ratio mismatch, discrete parts, asymmetrical parasitics	Use matched network; specify ratio tolerance; mirror layout
Output drifts with temperature	TCR mismatch, thermal gradient, self-heating differences	Specify TCR tracking; improve thermal symmetry; reduce dissipation
Unit-to-unit variation	Multi-vendor mixing, lot variation, assembly stress	Single network part; controlled sourcing; define alternates strategy
Unexpected noise floor	Resistor thermal noise, too-high values, poor filtering	Lower values where possible; validate bandwidth; add symmetric RC filtering

Resistor network being temperature-tested to illustrate TCR tracking importance.

RFQ-Ready Checklist for Differential Amplifier Input Resistors

If you copy/paste this into an RFQ, suppliers can quote comparably and you avoid “same value, different performance” surprises.

Item	What to specify	Why it matters
Resistor structure	Discrete vs matched network (array)	Thermal tracking and ratio stability
Ratio tolerance	Max ratio error for paired ratios (e.g., 0.01%)	Sets practical CMRR ceiling
TCR tracking	Ratio tracking in ppm/°C	Prevents CMRR collapse over temperature
Long-term drift	Stability spec over time / humidity	Avoids calibration creep and field variation
Package / footprint	Size and array footprint constraints	Affects layout symmetry and thermal gradient
Alternates plan	Approved alternates and re-qualification rules	Prevents silent CMRR regression in sourcing swaps

Recommended Models (Searchable Part Numbers, No Brand Mention)

Below are searchable model families / part-number formats commonly used as matched resistor networks or precision arrays in differential amplifier input stages. They are widely referenced online, but you should always verify ratio tolerance, TCR tracking, package, and availability for your exact design.

Model / Series (searchable)	Typical use	What to check before purchase
LT5400	Ultra-precise matched resistor network for high CMRR / gain accuracy	Ratio tolerance, tracking ppm/°C, package, resistor values
ACAS 0612 (thin-film array)	Compact matched array for differential input / divider networks	Array topology, ratio tolerance, TCR tracking, pinout
CAT16 (resistor array)	General resistor networks; use only when ratio needs are moderate	Whether it is matched or just bussed, tolerance, temp behavior
YC124 / YC164 (array codes)	Multi-resistor arrays often used for gain setting and input balancing	Topology (isolated/bussed), matching grade, package
RN73 / RG series (precision thin-film singles)	Discrete precision option when networks are not feasible	Lot control, placement symmetry, TCR, long-term drift

Important boundary

“Resistor array” does not automatically mean “matched network.” Some arrays are only convenient packaging with no ratio guarantees. Always check if the datasheet specifies ratio tolerance and tracking — otherwise your CMRR assumptions will break.

Bench scene illustrating how higher resistor values increase noise and bias-related error.

FAQ: Differential Amplifier Common Input Resistors

What are “common input resistors” in a differential amplifier?

They are the resistors at the amplifier inputs that set input impedance and form the resistor ratios that determine gain and CMRR. In many 4-resistor differential amplifier designs, their matching to the feedback resistors is what decides how much common-mode signal leaks into the output.

Is 0.1% tolerance enough for high CMRR?

Not by itself. Tolerance describes initial value. High CMRR needs ratio matching and stable TCR tracking. Discrete 0.1% parts often do not track well thermally, so CMRR can degrade across temperature and between units.

When should I use a matched resistor network instead of discrete resistors?

Use a matched network when you need high CMRR (commonly ≥80 dB across temperature), low unit-to-unit variation, or you want to reduce validation risk. Networks typically offer better ratio tolerance and thermal tracking because resistors share a substrate and see similar temperature.

Why does layout symmetry affect CMRR?

Even with matched resistors, asymmetrical routing creates different parasitics and temperature gradients on each input leg. That changes effective ratios and converts common-mode interference into differential error. Mirror geometry and thermal symmetry are essential for repeatable CMRR.

What should I include in an RFQ for these resistors?

Specify resistor structure (discrete vs network), ratio tolerance, TCR tracking, drift/stability expectations, package/footprint, and an alternates plan. Without ratio and tracking requirements, quotes cannot guarantee CMRR performance.