KeithleyUpgrade

Keithley 199 Digital Board Replacement

This is an upgrade replacement for the digital control board on Keithley 199 multimeters.

The Keithley 199 is a digital multimeter built in the nineties. It is capable of current, voltage and resistance measurements and also provides a gpib interface for remote control.

These multimeters contains three electronic circuit boards, one which contains the analog circuitry, including analog signal conditioning and analog digital converter, a display board which contains the user interface, and a digital control board, with a processor who controls display, gpib interface and analog digital converter.

This repository contains hardware and software files needed to build a replacement of the digital control board.

Features

• Voltage Measurement 0.3, 3, 30, 300 VDC/VAC 6.5 digits
• Current Measurement 30mA, 3A DC/AC, 6.5 digits
• Resistance Measurement 300-3GOhm, 6.5 digits, 4 wire.
• Arbitrary integration period
• SCPI interface
• Free Software

Building the hardware

The hardware files are in Kicad format. Gerber files are (not yet) available.

Building the software

The software was built with the Microchip Toolchain.

We use Ceedling for building and testing (but for comfort, MPLABX files are available).

In order to build the software, go to the root directory and do:

$rake release

Or to perform the unit tests:

$rake test:all

Hardware

The hardware is pretty basic. It contains a pic32mx controller which do the heavy lifting. An ftdi chip provides an usb interface, while two open drain drivers controll the led displays. The external trigger input and output are connected to the microcontroller through the necessary input/output protection.

Calibration

TODO

Principles of Operation

The interesting part is the Control of the analog board. The analog board contains a series of switches that are controlled with a shift register. The adc conversion is performed with a discrete charge-balance adc which returns a pulse train with a pulse count proportional to the input voltage. The control of the adc is performed by the microcontroller in the digital board and should observe a precise timing in order to guarantee adc accuracy.

As shown in the figure above, the charge balance phase starts after the controller sends a start of conversion to the analog board. While the charge balance phase takes place, the controller counts the number of pulses it receives from the analog board. The analog board only send pulses on the up parts, thus the number of pulses is proportional to the charge acumulated. After exactly 16.66 msecs the controller sends an stop of conversion and the single slope phase starts, discharging the integrator. While discharge takes place, the controller counts the number of pulses received.

Licence

 * Copyright (c) 2015, Diego F. Asanza. All rights reserved.
 *
 * This software is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301  USA
 *