|
Out in Front |
 A
UNIVERSAL AMPLIFIER FOR VARIED INPUTSI am an amateur working on an 8051 microcontroller-based 96-channel scanner. I will be using an ICL7135 A/D converter. The scanner will have at its input a thermocouple or an RTD or a 0 to 10V, 1 to 5V, or 4- to 20-mA current loop. I want to design a universal amplifier that can take care of all these varied inputs. Please advise. Sincerely, |
Bill Schweber, EDN's technical expert in analog technology,
responds:
Sensor data acquisition across many
channels and for many sensor types always presents the designer with some
complex trade-offs. You'll need a multiplexer, of course, and for this number
of channels you'll undoubtedly use submultiplexing, typically in groups of
eight or 16 channels. You have to watch your error budget very carefully through
the various gain and multiplexer stages, though, so that offset, temperature
drift, and supply-sensitivity errors don't corrupt accuracy.
Next is your challenge of handling the wide mixed-signal levels and types. Are the channel-sensor assignments fixed, or does the mix change? If they are fixed, you could build dedicated signal-conditioning circuitry for the appropriate sensor at each input position. The circuitry for thermocouples is very different from that for high-level signals and has to include cold-junction compensation and, perhaps, linearization. The RTD circuitry needs a current source and may need lead compensation. You may also need isolation for some channels to eliminate ground loops and common-mode effects, as well as provide circuit and operator protection. Vendors of op- and instrumentation-amp ICs have extensive application and design information showing how to build these sensor-interface circuits. Although the bandwidths are low, there are many design subtleties. You need to use some sophisticated design and layout techniques to get accurate, consistent results.
If you need flexibility to change channel assignments, your best choice is to design removable modules functioning as a front-end circuit per channel to handle each signal type and isolation requirement. The output of each module can be a 0 to 10V conditioned output to the multiplexer. Many vendors offer these families of plug-in blocks that condition the various signal types and also handle the related compensation, linearization, isolation, and calibration issues.
"Ask EDN" solves nagging design problems and provides the answers to difficult questions. Address your letters to Ask EDN, 275 Washington St, Newton, MA 02158, fax (617) 558-4470, e-mail ednbos.mcimail.com.
| EDN Access |
feedback | subscribe
to EDN! |
| Table of Contents |