Core Performance Parameters of Lock-in Amplifiers

Requirements for lock-in amplifiers vary across applications: temperature detection requires low-frequency performance, RF applications demand high-speed response, biological tests focus on signal extraction capability under low SNR, and optical applications need current amplification for weak current signals. Below are the main standardized performance parameters of lock-in amplifiers.

Also called full scale sensitivity, FS characterizes the measurement sensitivity of a lock-in amplifier and has a voltage dimension. It is related to total system gain as follows:

FS = OUTmax / Atotal

OUTmax is the full-scale output value (e.g. 10V), and Atotal is the total system gain (e.g. 10⁷). For the example above, FS equals 1μV, which reflects the amplification capability of the system.

The output of a lock-in amplifier usually represents the RMS value of the input useful signal, and can be adjusted for application needs. SE series lock-in amplifiers support direct input of 1Vrms signals, with sensitivity from 1nVrms to 1Vrms calibrated in 1-2-5 sequence for easy adjustment.

OVL is defined as the input level at which any stage of the lock-in amplifier reaches overload or critical overload. Since weak signal detection usually deals with low-SNR inputs, overload often occurs at noise voltage spikes. OVL can be understood as the maximum allowable input noise level, i.e. the noise tolerance of the system.

OVL varies with different gain settings, so it must be specified together with its corresponding FS to be meaningful.

FS corresponds to the useful signal level at full output, while OVL refers to noise tolerance. OVL must be much larger than FS to fully utilize the signal extraction capability of lock-in amplifiers.

MDS is the minimum input signal that can be identified at the output, reflecting the system’s resolution for small signals. It is mainly affected by internal noise and temperature drift, and is defined as the minimum input at which the output stays stable within a specified fluctuation percentage.

For example, if a 100nV pure input signal remains stable within 10% error over long-term monitoring and across 20℃–30℃, and inputs below 100nV cannot meet this stability, the MDS is 100nV.

In China, MDS is usually defined by time drift only, while international standards define MDS based on both time drift and temperature drift.

Under a given FS (fixed gain setting), the total input dynamic range is the decibel ratio of OVL to MDS:

Total Input Dynamic Range = 20lg(OVL/MDS) (dB)

This parameter reflects the system’s ability to extract useful signals from noise. Higher resolution and larger noise tolerance result in a wider input dynamic range. The SE1022 has a total input dynamic range of >100dB, suitable for demanding noise environments.

Output dynamic range is the decibel ratio of FS to MDS:

Output Dynamic Range = 20lg(FS/MDS) (dB)

It represents the dynamic range of detectable useful input signals, within which the signal can be measured without being undiscernible or exceeding full scale.

Dynamic reserve is the decibel ratio of OVL to FS:

DR = 20lg(OVL/FS) (dB)

For example, 100dB dynamic reserve means the system can tolerate noise 10⁵ times higher than the useful signal. Dynamic reserve can be adjusted via gain allocation: lower front-stage AC gain prevents noise overload, and higher DC gain after PSD and low-pass filtering amplifies the signal to full scale.

With total gain unchanged:

• Higher AC gain + lower DC gain: PSD is prone to overload, dynamic reserve decreases, but output DC drift is smaller.
• Lower AC gain + higher DC gain: dynamic reserve improves with better anti-interference, but at the cost of output stability and measurement accuracy.

Dynamic reserve is related to noise frequency: it is 0 at the reference frequency, and increases as the noise frequency moves away from the reference frequency. Adding low-pass filter stages improves dynamic reserve near the reference frequency.

High dynamic reserve may introduce output noise and drift. In analog lock-in amplifiers, lower dynamic reserve means smaller output error and drift. In digital lock-in amplifiers like the SE1022, high dynamic reserve does not increase output error or drift, but raises output noise. In practice, use low dynamic reserve when possible.

Lock-In Amplifiers: Principle, Applications & Products | Saluki Technology

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