Hill-Rom Advanta P1600, Service Manual

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Theory of Operation
Chapter 3: Theory of Operation
Advanta™ Bed Service Manual (man168ra) Page 3 - 31
3
R63 is an R/2R resistor network whose output is 256 discrete voltage levels
linearly spaced from Vss to approximately 2.90 volts. The voltage at R63 pin
16 is determined by the D/A byte shifted into U2. R46 scales the range of R63
pin 16 to the 2.90 volts described above.
Resistor R20 acts as a current limiter, which protects U2 should a latch-up
condition occur. C12 provides decoupling.
Analog Multiplexer
Analog multiplexer U1 routes the selected reference or pressure transducer
output voltage to the amplifier. Resistor R19 acts as a current limiter which
protects U1 should a latch-up condition occur. C13 provides decoupling.
Amplifier
The amplifier is a two-stage, differential, amplifier circuit with a sum input at
the first stage and a difference input at the second stage. U6 has low input
offset voltage advantages while U5 has large output voltage swing advantages.
R1 and R2 act as voltage dividers that apply approximately 1.93 volts at U6 pin
10. This voltage is passed through the unity gain amplifier to U6 pin 8. The
differential voltages from the selected transducer are applied to U6 pin 3 and
pin 5. The difference between these two voltages is amplified by a gain of
approximately G = 256 and added to the 1.93 volts at U6 pin 8. This guarantees
a voltage within the operating range of the amplifier, including worst-case
component voltage tolerances and over-the-worst-case pressure range. The
voltage at U6 pin 7 is applied to a difference amplifier with gain of G = 2. The
voltage at U5 pin 3 is the output of the D/A converter whose value is that
needed to compensate for the DC offsets of the selected sensor and amplifier.
This voltage is subtracted from that at U6 pin 7, and the result is amplified by
G = 2. The voltage at U5 pin 7 is then applied to the A/D converter U4 pin 3.
Resistor R3 acts as a current limiter which protects U6 should a latch-up
condition occur. C1 and C3 provide decoupling.
Analog-to-Digital Converter
Analog-to-digital converter U4 can be addressed to operate in a differential
mode. This enables it to be configured to operate with higher accuracy.
Resistors R17 and R18 divide Vcc in half. This voltage is applied to the unity
gain amplifier U6 whose output is applied to U4 pin 4 (CH 1). To select U4, a
1 is applied to P131 pin 3 (A/D_D/A_CS). This 1 is inverted by U9 and applied
to U4 pin 2. A zero is applied to P131 pin 4 (DATA_IN) as a start bit resulting
in a 1 at U4 pin 13. A 0-1-0 clock pulse is then applied to P131 pin 14 (CLK).