Fluke ScopeMeter 123 Service Manual page 42

123
Service Manual
LF input
The LF-input (pin 42) is connected to a LF decade attenuator in voltage mode, or to a
high impedance buffer for resistance and capacitance measurements. The LF decade
attenuator consists of an amplifier with switchable external feedback resistors R131 to
R136. Depending on the selected range the LF attenuation factor which will be set to 1-
10-100-1000-10,000. The C-ASIC includes a LF pre-amplifier with switchable gain
factors for the 1-2-5 steps.
HF input
The HF component of the input signal is supplied to four external HF capacitive
attenuators via C104 and R108. Depending on the required range, the C-ASIC selects
and buffers one of the attenuator outputs :1 (HF0), :10 (HF1), :100 (HF2), or :1000
(HF3). By attenuating the HF3 input internally by a factor 10, the C-ASIC can also
create a :10000 attenuation factor. Inputs of not selected input buffers are internally
shorted. If required, optional FETs V151-V153 can be installed. They will provide an
additional input buffer short for the not-selected buffers, to eliminate internal (in the C-
ASIC) cross talk. To control the DC bias of the buffers inputs, their output voltage is fed
back via an internal feed back resistor and external resistors R115, R111/R120, R112,
R113, and-R114. The internal feed back resistor and filter R110/C105 will eliminate HF
feed back, to obtain a large HF gain. The C-ASIC includes a HF pre-amplifier with
switchable gain factors for the 1-2-5 steps. The C-ASIC also includes circuitry to adjust
the gain, and pulse response.
ADC output pin 27
The combined conditioned HF/LF signal is supplied to the ADC output (pin 27) via an
internal ADC buffer. The output voltage is 150 mV/division. The MIDADC signal (pin
28), supplied by the ADC, matches the middle of the C-ASIC output voltage swing to the
middle if the ADC input voltage swing.
TRIGGER output pin 29
The combined conditioned HF/LF signal is also supplied to the trigger output (pin 29)
via an internal trigger buffer. The output voltage is 100 mV/div. This signal (TRIG-A)
is supplied to the TRIGGER ASIC for triggering, and time related measurements (See
3.3.4 "Triggering").
For capacitance measurements the ADC output is not used, but the TRIG-A output pulse
length indicates the measured capacitance, see "Capacitance measurements" below.
GPROT input pin 2
PTC (Positive Temperature Coefficient) resistors (R106-R206) are provided between the
Input A and Input B shield ground, and the COM input (instrument ground). This
prevents damage to the test tool if the various ground inputs are connected to different
voltage levels. The voltage across the PTC resistor is supplied via the GPROT input pin
2 to an input buffer. If this voltage exceeds ±200 mV, the ground protect circuit in the
C-ASIC makes the DACTEST output (pin 24) high. The DACTEST line output level is
read by the D-ASIC via the slow ADC (See 3.3.2 "Power"). The test tool will give a
ground error warning.
Because of ground loops, a LF interference voltage can arise across PTC resistor R106
(mainly mains interference when the power adapter is connected). To eliminate this LF
interference voltage, it is buffered (also via input GPROT, pin 2), and subtracted from
the input signal. Pin 43 (PROTGND) is the ground reference of the input buffer.
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