Sensor Bridges and Instrumentation ADCs

Many precision sensors produce very small differential signals. Strain gauges, load cells, pressure bridges, RTDs, thermocouples, and shunt sensors often need excitation, low-noise gain, filtering, and an ADC that can resolve microvolts or millivolts without being fooled by common-mode voltage or interference.

Learning Objectives

By the end of this lesson, you should be able to:

explain Wheatstone bridge measurement;
describe excitation and ratiometric conversion;
choose instrumentation amplifier gain and input range;
understand why delta-sigma ADCs are common in precision sensors;
plan calibration, shielding, and verification steps.

Wheatstone Bridge Basics

A Wheatstone bridge converts resistance change into a differential voltage. In a load cell, strain gauges form the bridge and produce a small output proportional to mechanical load.

flowchart LR EXC["Bridge excitation"] --> BR["Sensor bridge"] BR --> INA["Instrumentation amplifier"] INA --> LPF["Low-pass filter"] LPF --> ADC["Precision ADC"] ADC --> FW["Calibration in firmware"]

Bridge output is often specified in millivolts per volt of excitation, written mV/V. A 2 mV/V load cell excited at 5 V produces only 10 mV full-scale.

$$
V_\text{FS}=S_\text{bridge}V_\text{EXC}
$$

For S_bridge = 2 mV/V and V_EXC = 5 V:

$$
V_\text{FS}=2\ \frac{mV}{V}\times5\ V=10\ mV
$$

Ratiometric Measurement

If the ADC reference is derived from the same excitation that powers the bridge, excitation changes can cancel in the ratio. This is called ratiometric measurement.

When excitation rises, bridge output rises and ADC reference rises together. The code remains nearly constant for the same physical load, assuming the bridge and reference paths are well matched.

Instrumentation Amplifier

An instrumentation amplifier amplifies a small differential signal while rejecting common-mode voltage.

Important specs:

Spec	Why it matters
input common-mode range	bridge output may sit near mid-supply
offset voltage	microvolt offsets become measurement error
offset drift	temperature changes zero reading
CMRR	rejects common-mode noise
input bias current	creates errors through sensor resistance
noise density	limits low-level resolution

Instrumentation amplifier gain must keep the largest expected sensor output inside the ADC range, including overload and offset.

Precision ADC Choice

Delta-sigma ADCs are common for bridge sensors because they offer high resolution at low bandwidth and can reject line-frequency noise. Many include programmable gain amplifiers, current sources for RTDs, differential inputs, and digital filters.

For fast force, vibration, or motor-current signals, a SAR ADC with a suitable amplifier may be better because latency matters.

Filtering and Shielding

Small sensor signals are vulnerable to mains hum, switching converters, radio transmitters, ground loops, and cable pickup.

Practical measures:

use twisted-pair or shielded cable for differential sensor leads;
place EMI filtering at the connector;
keep bridge excitation and sense routing symmetric;
use Kelvin sense for remote excitation when accuracy requires it;
low-pass filter before the ADC;
separate high-current returns from sensor returns.

Calibration

A bridge measurement usually needs at least zero and span calibration.

$$
\text{engineering value}=a\times\text{code}+b
$$

For load cells, zero calibration removes tare load. Span calibration maps a known weight to the correct reading. Temperature compensation may require additional coefficients or lookup tables.

Verification Steps

Measure bridge excitation at the sensor terminals.
Short or balance the input and record zero offset.
Apply a known load or resistance simulator.
Check noise-free counts over the measurement bandwidth.
Repeat with motors, relays, radios, and power supplies active.
Test overload and disconnect behavior.
Verify calibration storage and startup recovery.

Common Failure Symptoms

Symptom	Likely cause
reading changes when cable moves	shielding, strain relief, or connector issue
zero drifts with temperature	amplifier offset drift or bridge self-heating
noisy counts near motors	grounding, filtering, or common-mode interference
saturates at small load	gain too high or common-mode range violated
correct at zero but wrong at span	gain calibration or excitation error

Common Mistakes

Measuring a bridge single-ended instead of differentially.
Forgetting that mV/V output depends on excitation.
Using high gain without checking common-mode input range.
Ignoring digital filter latency in a control loop.
Calibrating with unstable references or unknown weights.

Summary

Sensor bridges and precision transducers need clean excitation, differential measurement, low-noise gain, filtering, and calibration. Ratiometric ADC design can cancel excitation variation. Instrumentation amplifiers and delta-sigma ADCs are common, but their input range, noise, drift, filter delay, and layout must be verified with the real sensor and environment.

Mind Map

mindmap root((Sensor Bridges)) Bridge Wheatstone network Output in mV per V Vfs equals sensitivity times excitation Differential signal Common mode present Front end Instrumentation amp Gain setting CMRR Offset drift Input noise ADC Delta sigma common Ratiometric reference Digital filter Latency check Practical checks Measure excitation Shield cable Low pass filter Zero and span calibration Overload test Common mistakes Single ended reading Gain saturation Ignoring self heating Poor grounding