Stanford Research Systems DS345, Manual

Page: 97 / 139

Calibration
6-6
CALIBRATION
The following procedures determine the values of the DS345's calbytes. Any
adjustments should be done before starting calibration. Allow the DS345 at
least 1/2 hour warmup before beginning calibration. The first calibration (the
5.00 V reference calibration) requires the DS345's top cover be removed. All
other calibrations should be done with the DS345 completely assembled and
1/2 hour of warmup after reassembly. When the new calbyte values are de-
termined they should be entered into the DS345's RAM. In cases where the
calbyte value is determined to be greater than 32767 enter the value = cal-
byte value - 65536.
5.00 V Reference Calibration
This procedure measures the value of the 5.0 V reference voltage that the
DS345 uses for its internal A/D converter (calbyte # 1).
1) Measure the DC voltage at U103 pin 1 (top board).
2) The new value for calbyte 1 is Calbyte 1 = 32768 * (DC voltage/5.00).
Clock Calibration
This procedure sets the frequency of the DS345's internal 10 MHz clock.
The procedure is identical for standard and optional oscillators. Be sure that
the DS345 has been completely reassembled and warmed up for at least 1/2
hour before this calibration is started.
1) Connect the DS345's 10 MHz output to the frequency counter input with a
50
Ω
terminator. Use the frequency standard as the counter's timebase.
2) Adjust the value of calbyte 0 so that the frequency is within 1 Hz of 10
MHz (0.01 Hz for optional oscillators). The range of calbyte 0 is 0 to 4095. If
the clock cannot be calibrated with a value in this range do the clock adjust-
ment procedure.
Attenuator Calibration
This procedure calibrates the DC value of the DS345's output attenuators. If
the current calbyte value is negative use the value = old c 65536 in
the following calculations.
1) Connect the output of the DS345 to a DC voltmeter. Do
not
use a 50
Ω
terminator. Set the DS345 to sine wave, 1 kHz, 0 Vpp, 5 V offset.
2) Record the DC voltage. Record this value, with a high impedance termi-
nation, as Vref.
3) Connect the 50
Ω
terminator and measure the DC voltage. The new value
for calbyte 6 = old calbyte 6 * Vref/(2 * Vdc).
4) Set the DS345 to 2.5 V offset. Measure the DC output value. The new val-
ue for calbyte 7 = old calbyte 7 *Vref/ (4 * Vdc).
5) Set the DS345 to 1.25 V offset. Measure the DC output value. The new
value for calbyte 8 = old calbyte 8 * Vref/(8 * Vdc).