PICOWATT: AVS47-IB Two-Stage Computer Interface for IEEE-488

Revised:6 October 2008
To the opening screen

Dividing the computer interface in two parts
AVS47-IB key features
Powerful macro commands
Command summary and specifications

IEEE-488 Interface for the AVS-47B

RF interference is the problem

The most difficult problem when measuring low temperatures with resistive sensors is that they are so easily heated by RF interference of any frequency. The measurement current is usually kept very low in order to avoid the self-heating error and sometimes a measuring power as low as 10E^-15 watts is desired. This is equivalent to 3 uV across a 10 kohm sensor, and any possible RF interference should stay below the dissipation power of the measuring current. One may suspect, that such an interference exists, if the sensor does not cool down to the expected temperature. It is, however, very difficult to verify such existence by measurement. Therefore, the best approach is to try to eliminate all possible sources of interference, and this applies especially to instrumentation that must reside near to the cryostat.

Low RF emission level was the major design goal with the AVS-47B AC Resistance Bridge, and this is why we have continued our tradition of using analogue techniques in the bridge circuitry. But the computer interface must be digital.
IEEE-488 is still the most popular of the computer interface standards in scientific instruments. This interface was designed to be quite fast, which made it necessary to use high currents and short signal rise times in data transfer. These, in turn, can cause RF emissions and high-frequency ground loops. Using the IEEE-488 interface is therefore in contradiction with the hope for minimum difficulties with RF.

Dividing the computer interface in two separate parts

Our solution to this was to divide the computer interface in two parts. The first part is a very silent low-speed primary interface inside the AVS-47B. This "Picobus" interface does not contain any digital intelligence. Conversion between Picobus and IEEE-488 protocols is made by an external interface unit, the AVS47-IB, which is the second part. This "secondary" unit" has its own enclosure, 21x15x6 cm, and it also has its own mains power supply. The two parts are connected by a shielded cable, and the signals in this cable can be filtered when they enter the shielded room. The standard length for the Picobus cable is 5 meters, but longer cables are also possible. Data between the two parts is transferred using a proprietary synchronous, serial protocol. Because of the synchronous operation, the bit rate can be arbitrarily slow, which makes filtering easy and allows for long cables.

This concept of "two-stage interfacing" includes also that

The AVS-47B can be connected to a PC computer using only the primary interface without any additional costs
The noisy IEEE-488 bus can be separated from the cryostat by a sufficient physical distance

AVS47-IB Key Features

Commands for input multiplexer, range, excitation, input mode, deviation reference, display mode
Averaging and statistics (average of N measurements, min/max/std)
Digital filtering (reading based on a long running average is obtained immediately)
Digital self-calibration (best accuracy also on low excitation levels)
Automatic scanning of several sensors at predetermined intervals
(saves a lot of labour, can be left to collect readings without computer supervision)
Buffering of data (the computer is free for other purposes)
Reading or printing the results either in real time or from the buffer
Commands for the TS-530A include the set point and all the PID parameters plus measuring the heater voltage and current
Self test (*TST?) covers both the AVS-47B and the TS-530A
Serves as a printer port for the AVS-47B even without a computer
IEEE-488.2 status reporting and common commands

Powerful macro commands

Many of the above "macro" commands are very powerful, and using them can make the control program in the computer much simpler.

For example, taking an average of 100 readings would need 40 seconds. But you can program the AVS47-IB to generate a service request on "OPeration Complete" (*OPC), give the "AVE100" command and let the secondary interface take care of the measurement.

Scanning is even more powerful, especially when using the autoranging feature of the AVS47-IB. Then you need not worry about possible overrange situations. Scanning is also safe in the meaning that the sensors are not overheated by mistake. The multiplexer, range and excitation are changed only when the bridge input is grounded. The scanning procedure includes delays for letting the bridge to balance before it starts to take readings. Individual excitation voltages and averaging lengths can be programmed for each sensor, and the results can be buffered and/or printed. If there is only one sensor to be scanned, the prodecure shrinks to measuring this sensor at pre-set intervals.

See also: