Comments on Distributed A-D System
By K. C. Seino
(1) Limitations of MADCs: The MADC-Is (12-bit, 100KHz) were designed in 1979, 122 of them have been built, and 112 have been in use. The MADC-IIs (14-bit, 50KHz) were designed in 1986, 111 of them have been built, and 88 have been in use. The MADC channels are literally multiplexed and have some limitations. For example, when a fast snapshot is done on one channel, other channels are frozen.
(2) Advent of new ADCs: Several years ago, Burr-Brown Corp. came up with new ADCs called ADS7805/7809 (16-bit, 100KHz, low power, low cost, small package). This has made designs with simultaneous ADCs possible, rather than designs with a multiplexed ADC.
(3) Survey results: 8 out of 12 those who had responded to the survey would want 100KHz Snapshot. For the resolution, 3 out of 12 would want 14-bit, and 3 out of 12 would want 16-bit. We should not neglect the fact. We should provide the three classes of ADC modules shown on the attached. The Medium modules may be more expensive than the Slow ones. However, for those who need 100KHz Snapshot with 14 (or 16) bit resolution and are willing to pay for it, we should provide it.
(4) Medium Class ADC module specifications:
16 channels, differential inputs, +/-10V input voltage
16-bit ADC, simultaneous
14-bit accuracy, guaranteed
100KHz samples/sec per channel
(5) Settling problem with multiplexed ADC module: If the Slow module is digitized at 160KHz, the cycle time is only 6.25 usec. Settling from the signal source, through 100FT cable, through the multiplexers, through the instrumentation amplifier and to the ADC may present problems.
(6) Physical chassis: I would like to put the medium class ADC module in its own chassis without adding other functions (i.e. digital I/Os, Fast Digitizer), in order to achieve its best performance (i.e. 14-bit accuracy, 16-bit resolution).
(7) Testing: I intend to thoroughly test the ADC boards and to keep records on them in order to guarantee 14-bit accuracy.