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5700A/5720A Series II Calibrator
Service Manual
2-118
OHMS SENSE HI is connected to INT SENSE HI by K16 and K2 on the Ohms Main
assembly. INT SENSE HI is connected to the SENSE HI binding post by relay K2 on the
motherboard which is the sense high side of the 10 kΩ external standard. The ratio of
this third reading to the difference of the first two readings is proportional to the ratio of
the two resistances. Thus by knowing the resistance of the external standard, the
software can calculate the value of the internal 10 kΩ resistance value.
2-182. Calibrating 10 KW and 19 KW
The next step in calibrating the ohms function is to determine the 10 kΩ and 19 kΩ
values on the Ohms Main assembly. This is done just like the 10 kΩ value on the Ohms
Cal assembly, except the 10 kΩ resistance on the Ohms Cal assembly acts as the
standard and is placed in series with 10 kΩ or 19 kΩ on the Ohms assembly.
This is done using K19 on the Ohms Cal assembly and K29, K20A and K27 on the
Ohms Main assembly for the 10 kΩ, and K29, K20B and K39 for the 19 kΩ value. The
source on the Ohms Cal assembly again outputs 10V. Again three readings are taken.
The first and second are the same as previously described. For the third reading, the
differential amplifier +input is connected through K1 on the Ohms Cal assembly and
through K18A on the Ohms Main assembly for 10 kΩ and K30 and K18B for 19 kΩ.
The software then calculates the 10 kΩ and 19 kΩ values using the ratio of readings and
the known value of the 10 kΩ on the Ohms Cal assembly.
2-183. Calibrating 100 KW
Once the 10 kΩ value is determined, the 100 kΩ value is calibrated by comparing its
value against 10 kΩ. The 10V source is connected across 100 kΩ. Since the 10 kΩ
resistance is used as part of 100 kΩ, the current goes through both.
Relays K33 and K19A on the Ohms Main assembly connect 10V OUT HI to 100 kΩ.
Relays K23 and K27 on the Ohms Cal assembly connects OHMS SENSE LO to RCOM.
OHMS OUT LO, DAC OUT LO, and 10V SENSE LO are connected to SCOM. Relay
K22 on the Ohms Cal assembly connects OHMS OUT LO to SCOM. OHMS OUT LO is
connected to DAC OUT LO by K31 on the Ohms Cal assembly, and to 10V SENSE LO
by K31B and K28 on the Ohms Main assembly.
Only two readings are required because this time there is no path resistance to subtract
out. DAC OUT HI and DAC SENSE HI are tied together and connected to the -input of
the differential amplifier by relay K12.
For the first reading, the +input of the differential amplifier is connected to the sense
side of the 100 kΩ point in the string through K1 on the Ohms Cal assembly and K17 on
the Ohms Main assembly. For the second reading, the + it is connected to the 10 kΩ
point by K18A. The software calculates the 100 kΩ value from the ratio of the readings
and the known 10 kΩ value.
2-184. Completion of High Resistance Value Calibration
Once 100 kΩ is determined, 1 MΩ and the other values up to 100 MΩ are determined in
a similar way. Using the same techniques, 190 kΩ is calibrated against 19 kΩ, and so
forth up to 19 MΩ.
Too much current noise is generated by U3 in the differential amplifier circuit for
accurate calibration of resistances above 1 MΩ. For these values, U1 on the Ohms Main
assembly (a low-bias current FET op amp) is switched ahead of U3.
The 1 kΩ and 1.9 kΩ resistance values are determined from the 10 kΩ and 19 kΩ
respectively.