Analog-To~Digital Converter Test (Adc Test); Fuse Test (Fuse Test); Analog Control Voltage System Test (Acvs Test) - Tektronix 11A52 Service Manual

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Analog to Digital Converter Test (ADC Test)
The amplifier contains an analog to digital converter (A/D) (U750) that reads probe code voltages, measures
termination resistor temperatures, and reads data from serial (Level
II)
probes. The ADC test works by
selecting an internal self-test mode in the AID chip, which causes
it
to convert a known reference voltage. The
result of this conversion is read back by the processor; if it does not match an expected value within a given
tolerance, the test is flagged as failed.
Test Failure
Failure of the test indicates a problem with the analog to digital converter IC, U750.
Fuse Test (FUSE TEST)
Each amplifier contains four fuses (F1004, F1003, F1002, FlOOl) to protect the power supplies for external probes.
These fuses are checked by the Fuse Test self-test/ diagnostic routine. The protected sides of the fuses are
connected to a resistor array that forms a simple digital to analog converter. The output of this array is
connected to one of the amplifier's channel analog to digital converter. The ADC (U750) changes the voltage
from this resistor array to a numeric value, which is checked by the diagnostic routine to ensure that it is
within an allowable range.
Test Failure
If the returned value is outside the allowable range, it indicates that one or more of the fuses are open. If the
fuse test fails, it is likely that any Level 2 probes connected to the amplifier will not work correctly.
Analog Control Voltage System Test (ACVS TEST)
The amplifier relies upon a refreshed sample and hold system to generate all internal variable control
voltages. This system is called the Analog Control Voltage System; ACVS for short. The ACVS consists of a
section of U600, the IC that contains the sequencing logic and refresh RAM, a precision digital to analog
converter, along with one or two (depending on the amplifier model) sample-and-hold farms.
The ACVS test exercises this system by first setting all the analog control voltages to their minimum values,
then selecting and measuring each ACVS channel using the amplifier's ADC to verify that each channel is at
the proper value. Each ACVS channel is then set to maximum, one at a time, and the ADC is used to measure
that channel, verifying that
it
went to the proper maximum voltage. All the other channels are then checked
to confirm that they stayed at their minimum values.
Because the ADC range is limited, this test does not give 100% coverage of ACVS functionality. However, if
ACVS channels are flawed, the calibration system and diagnostics should catch any flaws that the ACVS
diagnostic misses. The ACVS diagnostic is also set so that when the test is executed in looping mode, a
technician using an oscilloscope triggered on the SA_Start signal (available on the 3rd test point up from the
bottom (GND) test point on the Main board) may verify that the outputs of the individual ACVS channels are
properly moving between their minimum (approx. -1 V) and maximum (approx. +1 V) values when the test is
11A52 Extended Service Manual 4-9