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Delsys® Electrode TesterFunctional Description
The electrode/amplifier combination will henceforth be referred to as the DUT
- device under test. The DUT's step response may be measured by using the square-wave oscillator internal to the tester. The SYNC OUT connector outputs a large-signal square wave suitable to trigger a measuring device (e.g., oscilloscope) so that an ensemble average of the system response may be obtained. The user may also drive the DUT with an arbitrary waveform by selecting the external excitation source. External inputs should be approximately 1V peak-to-peak. The INPUT MONITOR OUT connector provides a buffered version of the input but scaled by 2. This is to compensate for the factor of two gain that occurs when the test signal is converted to a differential electrode drive. (Incidentally, in principle it should have been possible to obtain the step response of the DUT by applying an external square wave from a function generator. However, upon closer examination, the generator in the lab was found to produce lousy square waves. The signal generated internally by the tester is of better quality.) The BANDWIDTH SELECT switch sets the bandwidth of the tester to either 50 kHz or 150 kHz. (Approximate values. See performance tests below.) The smaller bandwidth is suitable when measuring sinusoidal response, as it will produce a slightly cleaner signal, whereas the larger bandwidth should be used when testing step responses. Note that with BW SELECT set to 50 kHz that the magnitude frequency response of the DUT may be evaluated up to 8 kHz with a maximum error of 0.1 dB; at this frequency the phase frequency response will have an error ~ 10 deg. (See this tester frequency response.) The SOURCE ATTENUATION adjustment allows one to attenuate the electrode drive into the low mV level. The ELECTRODE MONITOR OUT connector taps directly off of one of the two electrode excitation pads. The ELECTRODE REFERENCE must be connected to the DUT electrode reference lead. SchematicThe above schematic is available in PDF format and in its original PCB123 format. List of Components
Circuit DescriptionMultivibrator CircuitThe internal square wave test signal is generated by U4, a 555-based multivibrator that is enabled when CLR is tied HIGH via switch S2. The square wave frequency is 10 Hz (48% duty cycle) and of amplitude 0-2V. All values are approximate. See p. 9 of TS555 data sheets for design equations. The output of U4 is made available to the user as an external trigger signal. The circuitry downstream expects a signal of 1Vpp. Resistors R16-R22 and the -2.2V supply circuitry (U3, R12, R13, C8, C9, and C10) condition the raw pulse-train from U4 so that it spans ±500 mV at test point T1. The position of jumper K1 and settings of trim pots R17 and R18 are iteratively adjusted until the required signal profile is obtained at T1. (Except for the first jumper position of K1, which bypasses R16 and is intended for circuit debugging, the remaining lower positions select different attenuation ranges in order to accommodate the variability in output voltage between different timer chips. Note that it will not be possible to condition the raw pulse-train in the case where U4 generates an output that spans 0.35 - 1.5V, which is an extreme scenario.) Power SupplyBecause the electrode drive is to be in the millivolt range U4 is powered with a low-voltage supply (+2.2 V) generated by programmable voltage regulator U2, R10, R11, C6, and C7. See p. 6 of the LM317L data sheets for design equations. It was experimentally found that when U4 is powered with +5V that obtaining a clean attenuated electrode-drive signal was very difficult. Lowering the supply voltage helped. C4, L1, and C5 form a pi-filter to de-couple switching transients at U4's VCC pin from the main +15V supply. Differential Drive CircuitU1, R3-R7 and R9 convert a single-ended input voltage into a differential electrode drive. Since vo1 = -vo2 this circuit effectively provides a gain of 2. Schottky diodes D1 and D2 limit the output voltage of vo1 and vo2 to about ±400 mV; under normal operation these dioes are off. Capacitors C1 and C2 limit the bandwidth of the circuit. In order to understand how this circuit operates with the rest consider the following circuit snippet.
The system transfer function is as follows, which describes a 1st-order low-pass system with time constant Since R6 = R9 the circuit can only attenuate the input. If Rd1 = Rd2 = 22k6 then the gain of the system (vo1/vi) ranges from [0, 0.004]. Capacitance C may be used to limit the bandwidth of the circuit, but at present it plays no significant role. If S2 is set to pass through the pulse train then the worst case value for Rd1 = R1 + (combination of resistors R16-R23 at their maximum value). Hence, the range of Rd1 is 22k6 to 26k2 ohms. Since Rd2 = 22k6 and C = 56pF, the circuit has a break frequency of greater than ~23 MHz. On the other hand, if S2 is set to pass the signal applied at connector J5 then the source resistance would have to be greater than ~3600 ohms to create a lower break frequency.
DebuggingMeasuring the frequency response of the differential stage (U1)Move jumper K3 into upper position. This applies the input signal directly to the input of the differential stage. Input signals should be limited to pear amplitudes of ~100 mVpp to avoid activating D1 and D2. Tester PerformanceFrequency Response Between INPUT and INPUT MONITOR OUT
As expected the gain is 2 and flat to beyond 100 kHz. (Theoretically
the magnitude response should be flat to > 1.25 MHz.) Frequency Response Between INPUT and ELECTRODE
MONITOR OUT
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