Revised: 2022-11-22
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AVS-48SI Picobridge®
The AVS-48SI Picobridge® is a high performance, basically analog, cryogenic AC Resistance Bridge front end with a stepless analog PID Temperature Controller.
     The AVS-48SI needs a small CPU for taking care of the lowest level firmware routines. Following our tradition of making analog instruments that cause the minimum possible RF emissions, the CPU is located away from the bridge itself, preferably outside the shielded cryostat room. Galvanic isolation of the signals eliminates possible ground currents, whereas grounding the shielding braid of the interconnection cable to the conducting wall of the room provides total safety against RF leakage. If this cannot be made, the wire cable can be replaced by an optional 5- or 10-meter long optical fibre link.
     The CPU, which is based on the very popular Arduino Mega2560 card, communicates with the controlling computer using the asynchronous RS232 protocol in its simplest and most standard form. Therefore the Picobridge can be connected to any computer, operating system and programming language as long as they support serial communication. The computer needs not have a physical Com: port, because a virtual serial port can be easily created using a USB-232 converter.
     Controlling the AVS-48SI by serial commands is straightforward, and the behaviour of all commands and queries can be checked before programming by using a hyperterminal program or a supplied small LabView VI. For example, a command sequence "CH2;RAN3;EXC3" sets the bridge to measure sensor #2 on the 3kΩ range and 100μV nominal excitation. No instrument driver is needed: writing application programs is supported by over 100 commands/queries for full control of the bridge.

     For those, who do not want to write programs, a full-featured Resistance Bridge and Temperature Controller is available as a LabView Virtual Instrument. Both the serial commands and the LabView VI offer many unique functions that may not be needed in daily research work or routine. However, they can be invaluable if one tries to find out why results were not as expected.
     According to Picowatt's long experience, resistance bridges, like most scientific instruments, are used for years. Some of our bridges have been used over two decades. All measuring instruments are subject to aging and to changes in environmental temperature, so how can one know whether the readings are still within specifications? By sending instruments to their manufacturers on a regular basis for recalibration? The AVS-48SI offers a new convenient no-cost solution: It has a self-calibration system, which derives accuracy of all ranges and excitations from seven ultra-stable wire-wound resistors whose actual values have been stored in memory. This unambiguous procedure can be run without moving the bridge from its operating place and temperature, it requires only one serial command, or a single click when LabView is used. No external standards nor user intervention is needed. Use this feature frequently and you can always trust on good calibration.
     The AVS-48SI Brochure is a long list of functions and features. For specifications and more detailed descriptions about the hardware, serial commands and LabView programs please refer to the latest AVS-48SI User Guide.


Confidence in Cryogenic Resistance Thermometry       Discussion of various error sources, how to estimate them and how the AVS-48SI can help to avoid them or to reduce their effects. Explanations on how noise and leakage specifications have been measured (PDF).

Comparison of AVS-48SI and AVS-47B      The hardware features of the two bridges are compared in this 8-page PDF document. Features and behaviour the bridges, when they are remotely controlled, are not discussed because the 47B is basically a stand-alone instrument whereas the 48SI could be called an "analog bridge front end" that can be used only under a computer's supervision.

All Serial Commands in Alphabetic Order
     This file lists, and to some extent also explains, all the available serial commands. These commands are used also by the LabView programs that come with the bridge (PDF).

Block Diagram Explained
      This file explains the operation of the bridge with the aid of its block diagram. The PID temperature controller is not included (PDF).

Circuit schematics and circuit board layouts
      Complete schematics and circuit board layouts of the bridge. Does not include wiring between boards (ZIP file 2.2MB).

Analog Squre Root Approximation
      Taking a square root of the PID output makes the control function more linear because it eliminates the quadratic behaviour of the PH = RH * I2. A microprocessorless analog design requires an analog square-root circuit. In this application, the high accuracy of a calculated function is not necessary whereas stepless operation is needed (PDF).

Improved Analog differentiator
      This analog differential can increase the gain of the PID circuit by a maximum of 30dB. High-frequency response is decreased so that gain at 50Hz is 0dB. The derivator has 10 program-selectable logarithmically spaced gain steps (PDF).

Bipolar current source
      This voltage-controlled bipolar source of very low DC currents has a gain adjustment that does not affect offset nor output impedance. In addition, standard resistance values can be selected for the current-sense resistor, as the gain can now be adjusted into both directions.

Presettable analog integrator
      This paper describes a low-speed analog integrator, which can be preset to a voltage from an external source before starting integration. The integrator can also be latched for an infinite time by measuring its output and then holding it preset to this voltage.

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