As the values of input voltage noise, feedback resistance, and bias current are fixed, the main vari-
ables under control of the user are the input capacitance, and shunt resistance of the source. The
lowest noise levels and DC errors will be provided by detectors with low capacitance and high shunt
resistance.
Large detector capacitance will cause gain peaking and thus ringing of the output waveform follow-
ing step functions of the input. The junction capacitance of a semiconductor detector may be re-
duced by reverse biasing of the detector in order to increase the depth of its depletion region. This
will however, increase the DC errors observed since the bias voltage generates an error current
(known as detector dark current) when impressed across the detector shunt resistance.
Rs-232 Operation
The PDA-750 is equipped with a bi-directional “three-wire” Rs-232c interface. It permits the unit
to be remotely controlled via an external computer and will report readings and the unit’s status
on request.
An interface cable is supplied with each unit. Plug the RJ-14 (telephone style) connector into the
port on the unit’s rear panel. Plug the 9 pin D connector into the computer serial port. Using a
communications program (such as WINDOWS Hyperterminal) set the appropriate communica-
tions parameters. The protocol is 9600 N 8 1, i.e. Baud Rate of 9600 Baud, no parity bit, 8 data
bits, and one stop bit. The command structure is as follows:
-7-
Prefix
Command
Carriage
Return
P:
READLAST
CR
Command List
Function
P:READLAST
INVOKES THE SENDING OF THE CURRENT READING
P:STATUSRQ
INVOKES THE SENDING OF THE UNIT’S STATUS
P:USERNG0X
SETS RANGE (X = 0-6) (0 is the most sensitive range)
P:BS-14.00
SETS BIAS FROM -14.00 V TO +14.00 V
P:A/W1.000
SETS A/W VALUE TO 0.100 TO 1.000
P:SETBIASX
TURNS BIAS ON (X = 1) OR OFF (X = 0)
P:TURNA/WX
TURNS A/W ON (X = 1) OR OFF (X = 0)
CR is Carriage Return, hex 0D, or Enter on a PC keyboard. The significance of the characters is as
shown above. A correct command will be met with the response:
“OK”.
An incorrect command will be met with the response:
“Invalid Command”.
Note: The PDA-750 is configured to automatically transmit readings following each measurement
without the need for a command. This occurs at a rate of approximately six readings per second. To
turn off streaming, send an XOFF character. To resume streaming, send an XON control character.
Содержание PDA-750
Страница 6: ...4 PDA 750 Front Panel Controls...
