EXTERNAL A TO D CONVERTERS
16-bit 4-CHANNEL A TO D CONVERTER
The 8 channels of 10-bit A to D on the TDS2020F can be augmented with others. Add Burr-Brown's ADS7825 chip to the TDS2020F to get a 16-bit 4-channel low-power A to D converter. It operates from a single +5V power supply that can be taken from the TDS2020F. Signal inputs are +10 to -10V and the reference is generated internally making this an easy chip to use. You should obtain the Burr Brown data sheet and check that the device meets your requirements in all details before using this chip and the support software. There are two files, for parallel and serial connection:
q #7825PAR.TDS implements a fast parallel connection
q #7825SER.TDS is an SPI-bus serial interface, slower but with fewer interconnections.
The files detail the pin connections required. The control lines are easily moved to other TDS2020F port bits just by changing the constants in the software.
The parallel conversion can capture data at up to 40,000 conversions per second.
12-bit 8-CHANNEL A TO D CONVERTER
Interface for MAX186 A to D converter
MAX186 from Maxim adds 8 analog to digital channels and gives you 12 bits resolution. The interface is serial so conversion times are more like 500�s than the 21�s you're used to on TDS2020F itself. However this is quite fast enough for many data collection and control applications. All the connection details and software you need to drive one or two MAX186 devices are in the library file #MAX186.TDS supplied with Update Service. As written the inputs are unipolar for 0V to +5V single ended inputs. However small parameter changes will give 4 or 8 differential inputs and the voltage range can be made bipolar for +2.5 to -2.5V inputs. You should obtain the Maxim data sheet before making changes.
8-bit 4-CHANNEL A TO D CONVERTER
PCF8591 has four 8-bit A to D converters and one 8-bit D to A. The A to D converter can be single ended or used in groups in a differential mode.
These chips are very useful for low cost A to D and D to A because you can connect up to 8 devices to just 2 wires of the I2C bus from the TDS2020F. If necessary they can be a few metres from the computer board without having to add buffers. You may need a pull-up on the I2C data line, see I/O EXPANSION ON I2C BUS, page 299.