Category Archives: AIOM – Analog Input Output Module

AIOM: updated schematics, differential input bias, high-impedance input protection and amplifier

Some progress, with the universal analog input output module, analog response analyzer, or line tracer, digitizer – however you want to call it. Added a few features – a bias supply for the differential input, and a high impedance amplifier (and a bit of protection circuitry) for the single-ended input.

First the bias supply – to allow a wide range of input voltages, say +-20 V (even below ground and well above the positive rail), we need a resistive divider network, and this needs to convert a differential voltage (with absolute voltages centered around ground), to a differential 0 to 2.5 V input, centered around 2.5 V (the internal bias of the ADC). A little calculator is used to find the right resistors and bias voltage. About 2.65 V will do the trick, for the desired input range, and resistor combination. Bias current is just about 1 mA or so, easily sourced/sinked by a OPA703 opamp.

adc input bias calculator

This input is mainly designed to sense low impedance sources, e.g., current shunts or supply voltages (lead compensation, or similar configurations). So the ~50 k input impedance will be perfectly fine.

aiom schematic 1 of 2

aiom schematic 2 of 2

The general circuit, nothing really exciting about it – the AD7712 has a 8 MHz crystal, will run easily up to 1 kHz conversion rate. But mostly, it will be run at 50 or 60 Hz, to suppress any mains related noise, or even at 10 Hz.

A quick test showed that the USB communication is working (using a JY-MCU ATmega32L minimum board) – just waiting for some long waiting times and train travels to write a simple user interface, to control the outputs and the data acquisition by Windows GUI. For plotting and data analysis, I will resort to gnuplot and/or R, not re-inventing the wheel here. Maybe a simple preview screen.

AIOM – Analog Input Output Module: the low-voltage low-current SMU

Quite frequently, I encounter rather large stacks of system power supplies and multimeters to test various types of equipment or circuit prototypes. Often, voltages are just in the 0..10 V range, sometimes, up to 40 V, at small currents. The response of the ciruit is then measured, often, after conversion to a voltage, or as a frequency, etc, with a counter.
This all works, but consumes space, a lot of power, heavy lifting, etc.

Therefore, what is needed, is a kind of Swiss Army Knife, a ADC-DAC unit, with some capable software, a small box that replaces at least two power supplies, and two voltmeters. Preliminary name: AIOM

Essentially, a small brother of the now very common source measure units, aka, “SMU”s.

Possible uses include:

(1) Test of diodes etc – trace recorder.

(2) Control of sweep generators, synthesizers, analog spectrum analyzers (most of these devices have a 0..10 V tuning-frequency control input, and often, a chart recorder-analog output, +-5 V, +-10 V, +-1 V, or similar).

(3) Test of VCO tuning curves, YIG current drivers, YTF circuits. Special VCO analyzers exist, but no need to block a whole lot of space just for some tuning test, and are too expensive to have multiple units at hand all the time. Often VCO tests need to be done at various temperatures, and over considerable time, to ensure performance of a circuit – multiple small test units will allow parallel testing, at very resonable cost.

aiom scheme draft


(1) Two 0..10 V outputs, 16 bit resolution, low drift, better than 0.05%; source and sink. Needs to have reasonable low noise.

(1b) Optional, to be added, voltage to current converter (to control current, rather than voltage, 0..10 mA, source and sink).

(2) Two inputs, one referenced to ground, one fully differential +-10 V.

(3) Ramp generator and similar features (rough sine, triangle, square/PWM), in hardware, to allow fast sweeps or steps or PWM-related tests (something like a simple arbitrary function gen).

(4) While the basic design will be kept strictly unipolar, there will be a simple switching matrix, to allow polarity reversal. This will also ensure fully symmetrical tests, e.g., for charge counting-battery test, charge-discharge efficiency tests.

The whole little thing has two parts: the AD/DA converter board (prototype build), and a switching materix (still waiting for some parts).

aiom board

First tests successful, stay posted!

aiom test setup