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Theory of Operation (2635A)
Detailed Circuit Description
2A
2A-25
• The open thermocouple detect circuitry is protected against voltagetransient damage
by A3Q14 and A3Q15.
• When measuring ac volts, the ac buffer is protected by dual-diodeclamp A3CR1 and
resistor network A3Z3.
• Switching induced transients are also clamped by dual-diodeA3CR4 and capacitor
A3C33, and limited by resistor A3R33.
2A-51. Input Signal Conditioning
Each input is conditioned and/or scaled to a dc voltage appropriate for measurement by
the a/d converter. DC voltage applied to the a/d converter can be handled on internal
ranges of 0.1V, 0.3V, 1V, or 3V. Therefore, high-voltage dc inputs are scaled, and ohms
inputs are converted to a dc voltage. Line voltage level ac inputs are first scaled and then
converted to a dc voltage. Noise rejection is provided by passive and active filters.
2A-52. Function Relays
Latching relays A3K15, A3K16, and A3K17 route the input signal to the proper circuit
blocks to implement the desired measurement function. These relays are switched when
a 6-millisecond pulse is applied to the appropriate reset or set coil by the NPN
Darlington drivers in IC A3U10. The A/D Microcontroller A3U9 controls the relay drive
pulses by setting the outputs of port 6. Since the other end of the relay coil is connected
to the VDDR supply, a magnetic field is generated, causing the relay armature and
contacts to move to (or remain in) the desired position. Function relay states are defined
in Table 2A-5.
Table 2A-5. Function Relay States (2635A)
Relay Position
Function A3K17 A3K16 A3K15
DC mV, 3V,Thermocouples Reset Set Set
DC 30V, 300V Set Set Set
ACV Set Set Reset
Ohms, RTDs Reset Reset Set
Frequency Set Set Reset
2A-53. DC Volts and Thermocouples
For the 3V and lower ranges (including thermocouples), the HI input signal is applied
directly to the A3U8 analog processor through A3R11, A3K17, and A3R42. Capacitor
A3C27 filters this input, which the analog processor then routes through S2 and other
internal switches, through the passive filter, and to the internal a/d converter. The LO
SENSE signal is applied to A3U8 through A3R35 and routed through internal switch
A3U8-S19 to LO of the a/d converter.
Guard signals MGRD and RGRD are driven by an amplifier internal to A3U8 to a
voltage appropriate for preventing leakage from the input HI signal under high humidity
conditions.
For the 30V range, the HI signal is scaled by resistor network A3Z4. Here, the input is
applied to pin 1 of A3Z4 so that an approximate 100:1 divider is formed by the 10-MΩ
and 100.5-kΩ resistors in A3Z4 when analog processor switches S3 and S13 are closed.
The attenuated HI input is then sent through internal switch S12 to the passive filter and
the a/d converter. Input LO is sensed through analog processor switch S18 and resistor
A3R34.