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Electronics -- USB Live Oscilloscope: ADCV Board

ADCV Board Circuit

Download USB-Live-Osci: Analog-to-digital converter board circuit schematic:
PNG image (843x604 as seen below): adcvboard.png (22kb)
High-quality PDF: adcvboard.pdf (95kb)
Permission to copy and use this schematic is hereby granted provided credit is given where it is due.

USB-Live-Osci: Analog-to-digital converter board circuit schematic [22kb]

Description

The analog input is first fed into a fast non-inverting buffer amplifier (OPA353) set up for unity gain which protects the analog-to-digital converter from out-of-range voltages.

The ADS830 is a fairly cheap 8-bit analog-to-digital converter which supports sampling clocks from 10kS/s to 60MS/s. It has sort of a 3-stage pipeline meaning that the conversion result available on the digital outputs corresponds to the analog voltage 3 clocks earlier. Since we're sampling on a constant 10MS/s rate (remember: for lower "sampling rates", the sampling clock is still 10MHz but we select only certain samples before committing them to the USB-FX2 FIFO) this introduces a constant delay of 300ns. This is nothing to worry about most of the time but when comparing the time from digital and analog inputs, it must be remembered that the analog signal lags somewhat behind.

The digital outputs from the AD converter are fed into a latch (IC2) for 3.3V level conversion and then leave the board on the ADCV_DATA connector.

The AD_CTL plug not only provides the sampling and latch clocks (they have the same frequency but opposite phase) but also the config for the AD converter allowing to set the sensitivity to 1Vpp or 2Vpp or choosing between internal and external reference. (There is currently no extra plug for an external reference.)

Analog Input

The analog has to be attached to the CON-BNC connector. It must be understood that the input is DC coupled and demands a common mode offset voltage of 2.5V. This is ideal for 5V single-supply amplifier circuits which operator symmetrically around 2.5V. However, this is not what one expects when connecting to a "real" oscilloscope. If different signal levels are required, an extra input amplifier and level shifter has to be built (which will probably require an external power supply!).


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