Analog front ends for
low-level measurement.
Input impedance, first-stage noise and switch leakage determine what a measurement can resolve. The converter downstream records that result; it cannot improve on it.
The front end sets the resolution
The sampling specifications a datasheet prints (bits, rate, bandwidth) describe how faithfully an instrument records a signal that has already reached the converter. They do not describe what the signal lost getting there.
By that point the input impedance has loaded the source, the first resistor has contributed Johnson noise, and any switch in the path has contributed leakage current. A converter records those contributions; it cannot remove them.
So the front end is the stage that sets the achievable resolution, and digitisation is the stage that has to preserve it.
Front-end blocks
Amplification and photodetection, built as separate blocks so they can be combined for a given measurement rather than bought as a fixed instrument.
Photodetectors, single and balanced
Si PIN diodes · 320 to 1,000 nm · 1 MHz bandwidth
Gain: 10⁵ V/A fixed, or 5×10² to 10⁵ V/A variable
Silicon PIN photodiodes into a transimpedance stage, in single and balanced configurations.
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Balanced puts two matched diodes into a difference amplifier, for measurements where the signal is the imbalance between two beams rather than the intensity of either. Single is a direct intensity measurement, where there is no reference arm to subtract. The variable gain version trades bandwidth against sensitivity across decades, so one detector covers an alignment scan at high optical power and the measurement that follows at low.
Voltage preamplifier, fixed gain
Fixed gain: 5×10², 10³, 10⁴ or 10⁵ · DC to 10 MHz
Built at one gain, specified at order rather than switched in use, so the transfer function is a single constant with nothing in the signal path to change it.
Current preamplifier, fixed gain
Fixed gain: 5×10², 10³, 10⁴ or 10⁵ V/A · DC to 10 MHz
Transimpedance conversion at one gain, specified at order rather than switched in use, for detector and photodiode currents where the range is known in advance.
Voltage preamplifier, programmable gain
In design1 mV to 12 V · ≤2.5 nV/√Hz · DC to 10 MHz
Programmable gain on a single-ended or differential input, with the range set in software rather than by a front-panel switch.
Current preamplifier, programmable gain
In design25 pA to 1 mA · ≤3 fA/√Hz · DC to 10 MHz
Transimpedance conversion for photodiode and detector currents, with the feedback element selected per range.
Where the limit sits
The question is not how good any one stage is, but which stage sets the floor, and whether that can be established without rebuilding the experiment.
Relay-switched front end
Relay-switched signal paths, guarded input islands, offset nulled by a DAC rather than a trimmer.
Under software control
Range, gain, bandwidth and offset settable from the acquisition script over Ethernet.
On the same board
Front end and converter integrated, so the calibration describes one system rather than a chain of boxes.
Self-characterisation
The instrument measures its own noise floor, selects the range, and reports the resolution available in its installation.
Tell us what you are trying to resolve
Send the signal levels, source impedance and bandwidth your measurement needs. We will tell you which stage sets your noise floor, including when the answer is that you do not need us.
Describe your signal chain