Special Project Note 9.0
Waveform Capture System (WCS)
Summary of the Analog Front End
Hardware Design and Simulation Results
August 14, 1992
The RD/Controls group is currently involved in the development of a Waveform Capture System (WCS) for the Fermilab Magnet Test Facility (MTF). This system will measure the magnetic flux characteristics of the superconducting magnets being built by the MTF. The WCS consists of several modules interconnected via the VXI bus. One of these modules is the Analog Front End. This module takes the signal from a probe that is placed in the magnet and does the necessary conditioning for input to a V/F converter module.
This document describes all of the work up to date done involving the design of the Front End and the results of the simulations used to verify the design. This design is for the prototype. The purpose of the prototype is to allow design ideas for the final system to be tested. The design of each sub-section will be discussed separately along with the results of its simulation. Unfortunately, due to a bug in the simulation software, we could not present the results of the full system simulation at this time. Finally, the final design of the prototype will be discussed.
The analog circuit simulation package used to verify this design is known as PSPICE, it is produced by the Microsim corporation. It is one the most widely used analog simulation packages available for personal computers. PSPICE is basically a derivative of the original Berkely SPICE. Transient response, frequency response, and noise analysis are some of the analyses PSPICE can perform and what we have used to verify and test our circuitry.
2. Structure of the WCS Analog Front End
A block diagram of the system is shown in Figure 1. The module contains two amplifiers, two filters, two automatic offset compensators and a VXI interface. The gain of the amplifiers is programmed via the VXI bus. The function of the module is as follows. The preamp takes the differential probe signal and amplifies it by a programmable amount of gain. The output of the preamp is single-ended. This signal is then fed into the preamp auto-offset compensator and the subtractor. The auto-compensator is really the most unique feature of this module. What the auto-compensator does, essentially, is "strip off" the dc offset voltage from a signal fed into it. The separated offset signal is then fed into a subtractor where it is used to subtract the offset from the main signal. By incorporating the auto-compensator, any dc offset introduced into the signal from the probe and preamp can be eliminated automatically. Since the auto-compensator is not in the main signal path, it will not contribute any distortion to the signal. The signal from the first subtractor is then sent to the second stage amplifier whose gain is also programmed. The signal is then presented to two filters. The first is a 720Hz notch filter. This filter removes the noise generated by the trans-rex power supplies used to power the magnets. The 5KHz low pass filter removes any high frequency harmonics present in the signal. The signal from the low pass is fed into another auto-compensator - subtractor combination to remove any offset introduced by the filters and the second stage amplifier. After the second subtractor, the filtered and amplifed probe signal is ready to be sent to the rectifier. Note that this prototype does not specifically include the recitfier at this time. However, it can easily be implemented using the design from the intermediate prototype.
2. Design Specifications and Ideas
The following is a list of the design specifications, ideas and goals that were used in the design of this module. Note that some of these values are approximate and changed as the design proceeded.
Analog Front End
Signal Type: Sine, Square, Triangle and Ramp
Frequency Range: 0.1 Hz - 10Hz (plus up to 30th harmonic)
Voltage Range: 100nV - 10V (p-p)
Frequency Range: 0.1Hz - 300Hz
Voltage Range: 0 - 10V (p-p)
Offset: < 50mV
Harmonic Distortion: < 0.001%
Noise: as small as possible
Size: one standard VXI module
Power Supplies: +/- 15V, +5V with isolated analog and digital grounds.
Progammability: Gain should be programmable via VXI bus.
Type: Instrumentation amplifier, AD625
Gain: 1, 100, 200, 500, 1000
Distortion: < 0.001%
Offset: As small as possible.
Type: DC, very low noise
Input: Bipolar, Single-ended
Output: Single-ended, high current
Gain: 10, 20, 30, 100, 200, 300
Center Frequency: 720Hz
Attenuation at center: > 90dB
Attenuation at 300Hz: < 0.01dB
Phase Shift: constant or zero
Low Pass Filter
Pass Band Frequency: 1KHz
Stop Band Frequency: 3KHz
Pass Band Attenuation: > 100dB
Stop Band Attenuation: < 0.001dB
Phase Shift: constant or zero
Function: Remove dc offset from input signal.
Accuracy: DV (Vos in - Vos out) < 50mV
3. Design and Simulation Results
We will now discuss the design and simulation results of each of the individual sub-systems starting with the pre-amp and auto-compensator.