HP 3457A Service Manual page 174

Current Input Path &
This path consists of R401 - R405, U403, R408 and U10! MOSFET switch 3. From the output of the
front/rear switch, the input current under test, AC or DC, is routed to the AMPS section. Input cur-
rents are switched to shunt resistors R401 - R405 through relays K401 - K403 and scaled (IRshunt) to
generate a 300mV full scale signal level. When changing amps ranges, the 1.0A range (R401) is always
enabled to provide a continuous current path at the input terminals while the next range configuration
is being selected. Once the new configuration is set, the LOA range switching (K401) is disabled. The
relay configuration for the various current ranges is shown in table 8-1.
For DCI the voltage is sensed by input hybrid UI01 at pin 40. The 1.0A range is sensed at the 0.1 ohm
shunt (R401) through analog switch U403. For all other DCI ranges the sense is made at R405 through
analog switch U403. For ACI measurements, the current shunts are configured identically to those for
DCI with the only difference being the range bounds and the sense path. ACI ranges are from 1A to
30mA. For ACI the voltage is sensed at the junction of R403/R404 with K402 pins 9 - 13 and routed
to the AC board (A3) through A2U402. From the AC board, the signal is routed through A2U403 pins
5-6 to input hybrid A2U101, pin 40,
On the 300uA current range the resolution for 6 digits is 1OOpA. This means that leakage currents from
the current input lead may be very critical to the 300uA offset specification. To minimize the leakage
current resulting from the clamp diodes, U401a is used as a bootstrap amplifier to force the voltage and
the leakage current across CR40! and CR403 to be zero.
Table 8-1 Relay Configuration
RANGE CONTACT CONNECTIONS RESISTANCE VALUES td]
K401 p4-6, 11-13
1.00 K402 p4-6, 11-13 0.1 OHM
K403 p13-9
300mA K402 p4-8, 9-13 1.0 OHM
K403 p4-8
30mA K402 p4-8, 9-13 10.0 OHM
K403 p4-6
3mA G02 p4-6;K403 p13-11 100.0 OHM
300uA K402 p4-6;K403 p13-9 1000.0 OHM
8-22. Autozero
The Autozero function is used to measure the offset (called a zero measurement) of the Input
Amplifier and subtract it from the input measurement. When the Autozero function is enabled. a zero
measurement is taken between each input measurement. The zero measurement is done in two dif-
ferent ways, depending on the function selected.
HP 3457A Multimeter
8-8