Kurt J. Lesker 100014434, Operation And Maintenance Manual

Page: 26 / 38

26 Series 902 Piezo
Analog Output
The 902 analog voltage signal are pins 5 (+) and 8 (-). Connect them to a
differential input voltmeter or analog-to-digital converter (ADC). The standard
902 analog transducer provides an analog output of 0 to 10 VDC.
Users can select either linear or logarithmic analog output with the Analog
Output Format – DAC command ( Set Up Commands section). The user can
also select linear and logarithmic outputs with a full scale output of 5 volts.
Do not connect the negative side of the analog output (pin 8) to
the power supply return (pin 4) or to any other ground. The
voltage drop from the supply current will produce errors in the
analog output voltage. The longer the cable, the worse the error
will be.
Linear versus Logarithmic Analog Output
Whether to choose linear or logarithmic analog output depends on the
pressure measurement needs of the user. Each analog output type has its
advantages. If using the optional Piezo display module, the display works
only with Linear 10.
Linear Analog Output
The advantage of the default 10 VDC linear analog output is that it is easy to
interpret, since the pressure reading (Torr) is equal to 100 times the voltage
reading (e.g., 7.600 volts = 760.0 Torr).
The disadvantage of linear analog output is that the voltage reading is only
1 mV at 0.1 Torr. This means the user must use a 16-bit ADC to read the
lowest decade of pressure.
Logarithmic Analog Output
The advantage of logarithmic analog output is that it has the same resolution
in each decade of pressure. The 902 has four decades of pressure
measurement. In a logarithmic analog output, each decade has 20% of the
full scale. This allows the user to measure all four decades of pressure with a
12-bit ADC. If it is necessary to see when power is on at vacuum, then the
logarithmic output is better (at vacuum, Logarithmic 10 reads 2.0 VDC and
Logarithmic 5 reads 1.0 VDC, but both linear outputs read 0.0 VDC).
The disadvantage of logarithmic output is that the pressure is more difficult to
interpret. The user will need to use a computer or calculator to read the
pressure equation. The equations are shown later in this section with the
Logarithmic 10 and Logarithmic 5 examples.