6.22 Front End Switch
In order to reduce current consumption and increase battery life, the F-2000 leaves the front-end circuitry off
except when it is time to take a reading. The "switch" that controls this is Q1. When the MCU needs to take a
reading, it biases Q1, bringing the front-end ground to true ground and applying power to U1, U2, and their
associated components. When the reading is complete, Q1 is "turned off" by the MCU. If Q1 becomes shorted,
excessive current draw will reduce the battery life. If Q1 becomes open, the F-2000 will display the "bAd CAL"
message since it will not be able to complete the self-calibration checks with the front-end off.
6.23 Signal Conditioning
The F-2000 probe contains a thermistor, which provides the input to a differential amplifier (U1). The circuit is
calibrated by VR1 to provide a 100 mv output when a resistance of 8343 ohms is applied to the probe input (see
Section 7 - Calibration). This value represents 85.0ºF, the low end of the F-2000's temperature scale.
6.24 Analog to Digital Conversion
The analog-to-digital converter (U2) is an eight-bit, four-channel serial device. The MCU provides the clock
(CLK) and chip select signals (CS), then uses the same bi-directional port bit (PCO) to clock out a/d converter
output (DO).
VREF is set by VR2 to the same value as the output of U1 when a resistance of 4746 ohms is applied to the
probe input (see Section 7 - Calibration). This resistance represents 110.0ºF, the high end of the F-2000's
temperature scale. This adjustment ensures that the thermometer can read values over its entire range.
The following are a/d converter channel assignments:
Channel O
Reads the conditioned probe input value from the output of U1. The analog value read here is
converted to a digital signal that the MCU uses to compute the values for the temperature
display.
Channel 1
Reads the 2.5-volt reference diode (D1). The value input here is used in the 5-volt power supply
evaluation (see next paragraph).
Channel 2
Reads the 5-volt power supply through a voltage divider consisting of two 47.5 K ohm resistors
(R8, R12). The resulting value (nominally 2.5 volts) is compared to the value read from the 2.5-
volt reference diode on channel 1. If the 5-volt supply has fallen out of regulation and the value
read on this channel is significantly less than 2.5 volts, the F-2000 displays the "Lo bAt"
message. An erroneous "Lo bAt" message could result if either the voltage divider components
or D1 is defective.
Channel 3
Reads the probe bridge before the signal is conditioned by U1. This is necessary to monitor
probe values outside of the 85.0º to 110.0ºF range. This input will detect shorted probe
(<approximately 2.2K ohms) and open probe (>approximately 18K ohms) conditions.
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