CIRCUIT DESCRIPTION
MODEL 635
Reference is made to the circuit schematic D13555. The amplifier module provides both a main AC/DC channel and an
auxiliary DC channel. Each channel incorporates a precision monolithic instrumentation amplifier in the first stage.
The first stage of the main amplifier (U1) is followed by gain adjust stages (U2,U3), filter stage (U4) and output buffer
stages U5, U6, and U7. The MONITOR output is provided through U6, a high-current (150mA) voltage follower. The
TAPE output is derived from U6, through the low-current buffer U7 (5mA). The output of U7 is also applied to the
comparator circuit which activates the voltage-limit LED. Retriggerable one shot (U9) times-out and extinguishes the
LED after the over-voltage condition no longer exists.
Unregulated +,-24VDC referred to the amplifier common is applied to three-terminal regulators U24 and U25, which
provide reg,-15VDC for the amplifier's op-amps. Another set of +,-24VDC, referred
to an isolated common, is applied to three-terminal regulators U22 and U23. These reg19V and -16V for the
auto-balance circuits. The fl24VDC is also applied to the adjustable-output regulator U21 and Q7 to supply the
module's bridge excitation voltage.
Relay switching is used to control the various modes and resistor values. Relay K1 selects EXT. CAL while K2 connects
the DC shunt CAL resistor, the completion resistors. Front panel rotary switching is used to select gain (S3) amplifier
mode (S2) and bridge configuration. (S5)
The AUTOBALANCE circuit is an electronic circuit which simulates the function of a balance pot by driving the balance
resistor with the voltage output of a DAC (U18). This output spans the range from -P to +P with a resolution of 1 part
in 4096. When the rear panel AUTOBALANCE BNC is shorted, the 12 bit DAC is set to zero, (-P) and then counts up
until bridge balance is achieved. The counter is then stopped and retains its status until another balance request occurs. An
on-board lithium cell, BT1, prevents loss of this status when the amplifier is off. If the battery is weak, the BAL LED will
light when the amplifier is turned on. This does NOT mean the balance has been lost, but the battery is due for
replacement.
When an autobalance cycle is initiated, the BAL LED will light until balance is achieved and then it extinguishes. If the
LED remains on after 2.5 seconds, an unbalanceable condition exists and must be corrected. Some common causes are
defective gages, wrong completion resistor setting, and no excitation voltage. It should also be noted that the gages must
be in a static (unloaded) condition during autobalance to prevent an erroneous setting. Also, if autobalance is attempted in
a non-bridge mode such as 1/4 DYN or EMF, the BAL LED will light and remain on.
Either the main channel or the auxiliary DC channel may be selected via the BALANCE REF SELECT jumper on the
PCB as the input to the balance comparator. (U16) The normal setting is on the DC channel, which is valid for all static
bridge modes, even when the main channel is in the AC or ZERO mode.
In order to maintain the isolation of the bridge excitation, an optical coupler (U12) is provided between the auto-balance
command and the auto-balance control circuit. Retriggable one-shot U13 reset the up-counter (U17) and the dual flip-
flop U14 turns on the Balance-Fault LED-1 if the bridge does not balance before U13 time-out.
Op-amp U26 provides a buffered reference voltage to the DAC (U18). The analog bridge-balance signal from U18 is
filtered and buffered by U19 and U20. The bridge-balance voltage at U20 is applied to the +S segment of the bridge
through balancing sensitivity resistors. (R131, R102) The primary operating voltage for the counter and the DAC is
applied through decoupling diode D12, R124 and zener regulator D17. If this voltage is lost during shut-down,
BT1supplies back-up voltage through decoupling diodes. (D13,14,15,16) This allows the counter and the DAC to hold
their digital status until power is restored.
Summary of Contents for 635-002
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