Low-cost ADC
by Martin Clausen (mc AT rotgradpsi DOT de)
This is a low-cost ADC for connection via parallel port.
Table of contents:
- Ultra low cost design
- 10KHz sample rate
- Two input voltage ranges
- Comfortable adjustable via PC software, trimmer free design
- Lines for measurement signal voltages on screen
- Bilingual user interface (German and English)
- Data can be streamed to hard disk.
- Helpfile included
- Program is freeware for non-commercial use.
80386, VGA, some RAM, about 100kB harddiskspace, parallel port, DOS or Windows (see this note)
Simply type adc and press enter. If you start the program for the first time or changed something, please enter adc /setup so you can check the configuration.
The input is protected through a 1M resistor and two diodes. The impedance of the input signal is then reduced by an operational amplifier. To allow positive and negative input signals, the level of the signal shifted to the middle of the range of the analog-digital-converter (ADC). To extend the input range a 100k resistor and a CMOS switch allows together with the input protection resistor to divide the input signal by the factor of eleven. A transistor and two diodes are employed to shift the level of the range signal from 5V/0V to 3.3V/-8V. This is necassary because the CMOS switch is powered from +/- 8V. The diodes make sure, that the transistor does not conduct while its basis is at voltages higher than 2.9 volts.
The ADC uses the successive approximation method. The PC starts the conversion and waits for the end of conversion signal from the ADC. The result is transferred in two parts via the 74LS157 into the PC. The 74LS14 is necessary to prevent latch-up and to reshape the signals.
See MiniDS for significant higher sampling rates (up to 20MHz).
- Semiconductors:
- 4 1N4151
- 1 BC558
- 1 4016
- 1 74LS14
- 1 74LS157
- 1 ADC0804
- 1 TL072
- 1 maybe: 78L05, 78L08, 79L08 + additional capacitors
Capacitors:
- 1 100pF cer
- 4 100nF cer
Resistors:
- 2 1k
- 2 10k
- 3 100k
- 1 1M
Mechanic:
- 1 sub-D socket, 25 pin
- 1 cabel, sub-D plug-plug, 25 pin
- 1 case
- 1 pbc
- 1 some thin wire
- 2 sockets for low voltage power connector
- 1 socket 8 pin
- 2 sockets 14 pin
- 1 socket 16 pin
- 1 socket 20 pin
- adc.zip DOS program V1.01 with source code (for Power Basic 3.20 with Power Tools 2.00), ini-file, dat-file
- adc.gif circuit V1.00
The part description in schematics is read as follows:
part type + value in exponential form + package description + subpart number in package
for example: C1040805,1 means Capacitor, 100nF, package SMD 0805, first subpart in package
Disclaimer: No warranty at all!
Copyright by Martin Clausen, Germany.
Contact the author: mc AT rotgradpsi DOT de
Impressum
