# Design of programmable scaling amplifier as a project for undergrad

I am building a custom microscope, PhD in biomedical engineering. I would like to have a scaling amplifier, with programmable offset and gain, that takes 0..10V signal X (from piezo element) and turns it into output -1..1V Y signal (into galvo mirror) such that Y=A*(X-B) where A~0.1-0.5 and B~-1..+1V are adjustable within 2-3 digits. In fact, currently we use A=0.18, offset B=+0.119

The frequency bandwidth should be around 1kHz or higher. The input signal is usually a sawtooth with 1Hz repetition rate. Input impedance should be > 10kOhm, devices outputs into 20kOhm impedance. The delay between input and output should be as short as possible, but 1ms is OK. I think we can tolerate output noise of 0.5-1mV.

Currently we are using rather expensive 1MHz scaling amp, so I want a copy of that with lower quality and much lower price tag. The problem is that we'll need roughly 4-5 of these, at around \$1000 a piece. I feel like there is a way to make these devices 10x cheaper.

Is that a feasible project for an EE undergrad? I have access professor with much more EE experience than I, for double-checking the final design.

I am considering custom PCB as a solution, or something arduino-based for programmable part.

• Do you know the impedance of the 0..10 source and the load on the -1..1? Also 3 digits of adjustment/precision is possible. However, 3 digits of accuracy is beyond the tolerances of regular electronics and can only be done with calibration. Have you ruled out 16b USB IO? Is there a latency requirement? – scorpdaddy May 4 '20 at 16:15
• @scorpdaddy re: accuracy. In effect i will do the calibration during alignment. I don't care if device says "B=0.001" but outputs "B=0.005" as long as these numbers are stable over 12 hours, does that make sense? – aaaaa says reinstate Monica May 4 '20 at 16:22
• Can you be clearer about what A can be and what B can be numerically. Also, is your input signal DC biased such that signal minus B doesn't go negative? What sort of noise output is acceptable for the design and how stable do the offsets and gain need to be drift wise and against temperature. – Andy aka May 4 '20 at 16:23
• @Andyaka thanks for the questions, I added more details. The temperature we can assume to be stable ~RT. – aaaaa says reinstate Monica May 4 '20 at 16:39
• As currently written, you state that the bandwidth should be 100Hz or higher, and the input to output delay should be of order a microsecond. Are these both correct? Because they don't seem to fit together. – Jack B May 4 '20 at 17:25

For signal fidelity (SNR), here are concerns

(1) plan on using a steel box, unless you can guarantee no magnetic fields (only steel handles the 60Hertz power and rectifier surge peak fields)

(2) plan on a Ground Plane; I'd default to 4" by 4" area, plus Power Supply; plan on a steel metallic strip between Power Supply and your analog circuits.

(3) assuming a goal of 0.1 millivolt PeakPeak random + deterministic "noise", with half of that for the Random, in 100Hertz bandwidth, the OpAmp noise density is

50uVPP / [ 6.2 sigma * sqrt(100Hertz) ] = 50uV/62

or

(4) assuming the output stage is gain-of-10, you are allowed 80 nanoVolt noise Density referred-to-input; that is total noise contributors of 400,000 ohms.

(5) to not be burdened by 1/F noise, the 1/F noise plot corner needs to be 50Hz or lower

(6) If this is knob-controlled, then use 10-turn pots with verniers.

If this is computer-controlled, then consider CMOS 10-bit or 12-bit multiplying DACs or DigiPots.

(7) Topology: do the math up front, followed by Gain_of_10X amplifier including a Low Pass Filter to set the noise (random and deterministic) floor.

(8) plan on +- 15 volt supplies, so life is easy. You only need about 1 watt total power, so the Ground plane easily removes heat. Maybe as low as 0.1 watt.

(9) stability over 12 hours: have heat paths from the Ground Plane (metal mounting posts that contact the Steel case and contact the Ground Plane, with metal screws). The thermal time constant of 1 cubic meter of copper is 11,400 seconds. The thermal time constant of 0.1 cubic meter (4") is 100X faster at 114 seconds. Having a central metal mounting post speeds that up by (2*2) to 32 seconds. That's probably acceptable.

(10) pick a good opamp: ensure it is spec'd by a plot for 1/F noise (I may have gone overboard on the achievable noise floor; your call); ensure opamp handles +- 15 volt rails (IMHO)

As with any free suggests, your mileage may vary.

(11) black brick power supplies (switching Regulators) will be a problem. You want to avoid Ground Loops (charges such as from the black_brick that MUST find a return path).