Fluke 5720A Feedback Loop, Current Output Switching, Generation of the 2.2A Range, Relay, 2.2 mA

Theory of Operation 2

Analog Section Detailed Circuit Description

Relay

Table 2-15. Relay Settings for Current Range Selection

Range

K5

K6

K7

K8

K9

S=SET R=RESET

220μA

R

R

R

R

R

2.2 mA

S

S

S

R

R

22 mA

S

R

R

S

R

220 mA

S

R

R

R

S

2-192. Feedback Loop

The output current develops a 2.2V full-scale voltage across the appropriate shunt resistors. Buffer amplifiers U2 and U3 isolate the shunt from the remaining feedback circuit. The negative feedback buffer is op amp U2 configured as a voltage follower. The positive feedback buffer is made of U3 configured as a voltage follower.

Both the feedback voltage from U2 and U3, and the input voltage from K1 and Q20-Q23 are applied to the precision dual 10:1 matched voltage divider network Z1. Any voltage difference between the two halves of the network is amplified by the dc amplifier on the heated substrate hybrid HR2. This amplified dc is applied to the complementary drive circuit and in turn to the transconductance amplifiers to complete the feedback loop.

Therefore, with a 22V full scale input and the 10:1 divider, the voltage across the shunt network is forced to 2.2V by the feedback loop. The 2.2V across the shunt is developed by the full-scale output current on any of the four ranges. By programming the input voltage over a 10:1 range, the output current follows with a 10:1 range. By switching the shunt resistors, four 10:1 ranges give a total output range of 20 μA to 220 mA.

2-193. Current Output Switching

Relays K10-K15 switch the output current for the various modes of operation required by this assembly. When K13 is reset, it switches in a dummy load (R14) to prevent transients during switching, and also for use during diagnostics. Non-latching relay K15 connects the return lines to the output whenever an output is called for. Non-latching relay K14 connects the output signal to latching relay K12. Relay K12 switches the output to the AUX CURRENT OUTPUT binding post while in the reset position, or to the OUTPUT HI binding post while in the set position.

The four ranges of output current can be connected to the B CUR line by relay K11. B CUR is routed to the rear-panel 5725A connector. This allows all current ranges to be available from the binding posts on the 5725A Amplifier if so selected by the operator.

2-194. Generation of the 2.2A Range

To generate the 2.2A range, the Current assembly is set to the 22 mA range with its output directed to the High Voltage/High Current assembly (A15). This connection is made via the IHV line by relay K10 in the set position. The High Voltage/High Current assembly amplifies current by 100 to create the 2.2A range. This high current output is returned to the Current assembly via the B CUR line. Relay K11 in the set position directs it to the output relays, K12-K15, of the Current assembly.

During internal calibration of the 2.2A range, the Current assembly is set to the 22 mA or 2.2 mA range and directed to the High Voltage assembly in the same manner as previously described.

2-125