UVP UVX Radiometer - Manual (Page 9)

UVX Radiometer 9
Radiometer Details
Circuit Description
This section describes how the UVX Radiometer operates. Three illustrations are included for use in
conjunction with this section. Figure 2 is a Functional Block Diagram.
Current to Voltage Converter
The silicon detector used in the sensor may be modeled as a current source. For maximum linearity, the
detector load should represent a short circuit. The input amplifier on the board meets this requirement
and converts the input current to a voltage that is then applied to the analog to digital
(A/D) converter. A
switchable 4, 40, or 400:1 voltage divider in the amplifier feedback loop allows the current gain selection
required to produce a readout directly in the appropriate units of measure.
A/D Converter
The A/D chip used in this circuit is a high performance, low power 3-1/2 digit converter, complete with
seven segment decoder, display drives, internal reference and a clock. This chip retains the high
accuracy of much more expensive units by providing an auto-zero to less than 10 uV, zero drift of less
than 1uV/°C, and roll over error of less than one count. The output of the chip drives the LCD directly.
Decimal Point Circuitry
The decimal point circuitry operates through a C-MOS chip to drive the appropriate decimal point as the
range is changed.
Low Battery Voltage Circuit
A low battery is detected by means of comparing a diode reference voltage to the trip-point of an
exclusive OR gate. When the battery voltage decreases to less than 7.0v, “LOBAT” text will be shown on
the display on 2014 and newer models, or a colon (“:”) for 2013 and older models.
HOLD/READ Circuit
The A/D chip is equipped with "hold" function pin which, when driven high (by the READ/HOLD switch),
holds the currently displayed reading.
ON/TEST Circuit
The ON/TEST circuit applies a one second DC signal to the back-plane of the LCD, forcing all the
segments to come on. After one second, the TEST portion of the circuit returns the unit to normal
operation.
Radiometer Circuit Calibration
This section refers to the calibration of the Radiometer, not of the sensor. UVP recommends recalibration
of the Radiometer every 6 months and provides calibration services for a nominal fee. Contact the factory
for details.
Note: Should the user attempt to perform a calibration on the Radiometer prior to the end of the warranty
period, then the warranty on the Radiometer becomes void.
Required test equipment for those who wish to calibrate their own Radiometer:
• Variable current with range from .005uA to 1.0uA. Note: See Fig 3 for a calibration current source
• Precision microammeter measures current source output
• 500 K ohm resistor (±0.1 %)
• 5 to 9 V DC adjustable power supply (10mA or more)
• 0 to 10V DC digital voltmeter
• Small Phillips head screwdriver
• Small flat head screwdriver for trimpot adjustment