Unfortunately the three EXT. inputs have pull-up resistors to the positive supply. Presence of the resulting voltage on the inputs poses a problem in most measurements. This drawback can be completely overcome by using suitable operational buffers which drag to zero the DrDAQ inputs: to be able to really go through 0V, the operational amplifiers need a negative supply. In this post I am presenting a complete buffer interface working off the 5 V taken from the DrDAQ itself and generating the required negative voltage at -2.5 V.
The buffer connections are all plug-in: therefore when sensors other than voltage sources are used, the corresponding plugs are removed and the sensor is plugged into the telephone type connector of the DrDAQ. (NOTE: in my DrDAQ the telephone plug EXT. 1 had a defective contact and I had to remove it, so check these sockets if you have erratic operation.)
This buffer provides the following facilities.
1) Eliminates the unwanted voltages at the EXT. inputs. The new buffered inputs are at a very high impedance and can go trough zero Volts thanks to the built-in negative supply feeding the operational amplifiers.
2) The extra operational amplifier can be used to increase the input dynamic range of any one EXT. input from 0 – 2.5V up to 0 – 10 V, but still providing a nearly infinite input impedance and also reading the input through zero Volts.
3) The extra operational amplifier can be used to provide other features, as shown below & in diagram.
4) Precision 2.49V standard voltage source for one point routine calibration of the DrDAQ. Also a three point calibration facility can be easily provided as shown.
The buffer works with the + 5V supply voltage supplied by the DrDAQ and normally does not require an external supply. It can also work with an external + 5V or + 12V power supply by simply removing a jumper. Reverse polarity protection is provided.
The unit is built on a small prototype perforated board. After assembly the board is secured to the wooden base with double edge tape. A subminiature in-line 24 pin connector (S1/J1) is used on one edge of the board for quick removal. External connections are made with miniature screw terminal M1.
The LM324, IC2, provides the four operational amplifiers. Amplifiers A, C and D work as unity gain followers for the three EXT. inputs of the DrDAQ. Resistors R3, R4 & R5 provide the Autodetect function for operation with .DDS files. The output of the astable square wave multivibrator IC1 (NE555) is rectified by voltage doubler D1-D2. The negative voltage has a value of -2.5V for a supply voltage of 5V. When the jumper between pins 11 &12 of M1 is removed and +12V is supplied to pin 12 by a external supply, negative voltage reaches the value of -9V.
The 2.49V precision voltage is supplied by special zener IC D4 (LM336). Specification for D4 calls for an initial voltage of between 2.390 and 2.590 V. Resistor R6/R7 must be selected with the help of a precision DVM to obtain 2.49 V. Then long term calibration of this source will keep within a few PPM for ambient temperatures 15 to 25 °C.
To calibrate a voltage channel, just connect it to pin 8 of S1 and read the voltage on Picolog. The channel can then be calibrated by writing a .DDS file or by writing table/equation in Picolog. For three point calibration just connect the precision resistor voltage divider shown to pin 8 of S1 to obtain 1.5 V and 0.5 V. Then a scaling table can be prepared.
The PC screen shows the DrDAQ reading the voltages of a Microprocessor controlled PSU on EXT. 1, 2 & 3.
All considered the DrDAQ is an amazing piece of equipment. In a small package, practically the size of a credit card, together with the Pico supplied software, it packs an Oscilloscope/Audio Spectrum Analyser, a PH /Oxygen meter, a Multi-waveform Function Generator, 4 input/output digital ports, a resistance measurement facility and 3 pre-settable analogue (voltage) inputs, not conting the built-in sound and light sensors. One of the most useful features is the Data Acquisition capability when working with Picolog software.
- REVISED SCHEMATIC & ERROR CORRECTED