Fluke 123B/S Service Manual - Page 51

3 Circuit Descriptions 3.3 Detailed Circuit Descriptions DACTEST output A frequency detector in the T-ASIC monitors the ALLTRIG signal frequency. If the frequency is too high to obtain a reliable transmission to the D-ASIC, the DACTEST output pin 29 will become high. The DACTEST signal is read by the D-ASIC via the slow ADC on the Power part. It and indicates that the D-ASIC cannot use the ALLTRIG signal (e.g. for qualified triggering). Current Source A current source in the T-ASIC supplies a DC current to the GENOUT output pin 1. The current is used for resistance and capacitance measurements. It is adjustable in decades between 50 nA and 500 µA depending on the measurement range, and is derived from an external reference current. This reference current is supplied by the REFP reference voltage via R323 and R324 to input REFOHMIN (pin 6). The SENSE input signal is the buffered voltage on Input A. For capacitance measurements it is supplied to a clamp circuit in the T-ASIC (pin 59). The clamp circuit limits the positive voltage on the unknown capacitance to 0.45V. The protection circuit prevents the T-ASIC from being damaged by a voltage applied to Input A during resistance or capacitance measurements. If a voltage is applied, a current will flow via PTC resistor R172 (on the Channel A part), V358/V359, V353, V354 to ground. The resulting voltage across the diodes is approximately -2V or +15V. R354/R356, and V356/V357 limit the voltage on the T-ASIC GENOUT output (pin 1). The BOOTSTRAP output signal on pin 3 is the buffered GENOUT signal on pin 1, or the buffered SENSE signal on pin 59. It is supplied to the protection diodes via R352, R353, and to protection transistor V356, to minimize leakage currents. On the ICAL-output of the T-ASIC (pin 5) a copy of the output current on GENOUT is available. The current is supplied to the Channel A C-ASIC via R144. ICAL shows the same time/temperature drift as the GENOUT measurement current, it can be used for internal calibration of the resistance and capacitance measurement function. Capacitor C356 is use for hum/noise suppression. Square Wave Voltage Generator For Probe Adjustment For probe adjustment, a voltage generator circuit in the T-ASIC can provide a 2.5Vpp, 760Hz, square wave voltage via the GENOUT output pin 1 to the Input A connector. Capacitor C357 is the external timing capacitor for the generator. 3-23