Capstone lab: a low-voltage sensor amplifier and filter

Turn the need into requirements

Assume a sensor produces 1.65 V plus a ±20 mV, 10 Hz signal, with 1 kΩ source resistance. The intended physical supply is 3.3 V. Choose noninverting gain 11 using 100 kΩ/10 kΩ feedback and follow it with 15.9 kΩ/100 nF low-pass. Assume a later receiving input is high impedance. The nominal signal output before filtering is 1.65 V ±220 mV.

At 10 Hz, the RC magnitude is approximately 0.995047. The ideal filtered signal is approximately 1.65 V ±218.910 mV. At 1 kHz, the filter magnitude is approximately 0.099600, so a hypothetical 1 mV interference at the sensor becomes approximately 1.0956 mV at output after gain and filtering. This is limited first-order rejection, not removal of all interference.

Reproducible educational model

Save sensor-capstone.cir and run ngspice -b sensor-capstone.cir with an available standalone executable.

Sensor deviation gain 11 with 100 Hz RC filter: ideal educational model
Vref ref 0 DC 1.65
Vs sensor ref DC 0 SIN(0 0.02 10) AC 1
Rsource sensor plus 1k
Rg minus ref 10k
Rf amp minus 100k
Eamp amp ref plus minus 100000
Rfilter amp out 15.9k
Cfilter out 0 100n
.op
.ac dec 50 1 10000
.tran 100u 500m 0 100u
.print ac vm(out) vp(out)
.print tran v(sensor) v(amp) v(out)
.save all
.end

The source Vs is referenced to midrail. Eamp is a finite-gain dependent source, without supply clipping, noise, GBW, slew, or bias current. Actual closed-loop gain is approximately 10.998790; use that when comparing high-precision simulation values. At the DC operating point, sensor, amp, and out are 1.65 V. Compare within 1 mV. After startup, the 10 Hz output peak deviation should be approximately 218.886 mV; compare within 2 mV.

Complete the physical design record

Before a build, select a part whose 3.3 V supply, input common-mode range near 1.65 V, output range 1.431–1.869 V, and expected load current are supported. Check gain-bandwidth, slew, input bias error through 1 kΩ, offset, noise, decoupling, and reference drive. Choose resistor/capacitor tolerances and assess corner extremes. The required sinusoidal slew is only approximately 0.01375 V/ms here, but that alone does not qualify a device.

Deliver a connection table, hand predictions, exact deck, result table, and limitations. Standalone model success does not establish ALcad engine execution or a physical sensor's safety or accuracy. See the ngspice documentation for the analysis reference.