Circuit Description; Average Responding Ac Convertor; (Option 001) - HP 3455A Operating And Service Manual

Section VII
8-54. Squaring Amplifier. The squaring amplifier is a log
amplifier circuit which takes the log of the input voltage, or
in this case since there are two transistors (Q9A and Q11A)
in the feedback loop, takes twice the log of the input
voltage. Therefore, the output of the squaring amplifier is
equal to 2 log IV inl or log | V inI?.
8-55. Square Root and Averaging Amplifier. The square
root amplifier reverses the action of squaring amplifier.
The input to the amplifier is through logging transistors
QIIB and Q9B. Output of the square root_amplifier is
equivalent to 1/2 oa |Vini? s log JT VinI?. The
operations of the square root amplifier and the averaging
amplifier are simultaneous and inter-dependent. The com-
bined output of the two circuits is a dc level proportional
to the rms value of the input signal.
8-56. AVERAGE RESPONDING AC CONVERTER
(Option 001).
8-57. General.
8-58. The average ac converter is an average responding cir-
cuit calibrated to the rms value of a sinusoidal input. Full
scale output of the converter is 6.6667 V de for all ranges.
Figure 8-20 shows a simplified schematic of the converter.
8-59. Circuit Description.
8-60. AC Input Attenuator. The ac. input attenuator is an
RC circuit which provides a fixed attenuation of 100-to-1
on the 100 V ac and 1000 V ac ranges. Attenuator switch-
ing is done by reed relays which are controlled by the
inguard controller. Input resistance of the AC Converter is
approximately 2 megohms.
INPUT.
ATTENUATOR
KL
CONVERTER
AMPLIFIER
AC FEEDBACK
SOK
THEORY OF OPERATION
Model 3455A
8-61. Converter Amplifier. The converter amplifier uses a
dual FET input stage to maintain a high input impedance.
An operational amplifier provides the necessary gain to
drive the output stage of the converter amplifier. The out-
put stage of the amplifier is a current driver circuit. Two ac
feedback paths provide fixed gains of 1 or 0.1. An integrat-
ing amplifier (U4) is used to maintain a dc level of 0 V dc at
the output of the Converter Amplifier. The integrating
amplifier also determines the low frequency cut-off point
of the Converter Amplifier. (The cut-off frequency is ap-
proximately 300 Hz on the FAST ACV mode and 30 Hz on
the ACV mode.) A diode protection circuit is used to limit
the output of the Converter Amplifier to approximately
+ 6 V peak to prevent saturation of the amplifier.
8-62. AC Ranging. AC ranging is accomplished by attenu-
ating the input signal and changing the gain of the converter
amplifier. The input attenuator provides a fixed attenuation
of 100 to 1. The Converter Amplifier has fixed gains of |
or 0.1. Table 8-1 shows the various combinations of amp-
lifier gain and input attenuation necessary for the input
voltage ranging. Full scale output of the Converter Amp-
lifier is approximately 1 volt rms for all ranges.
8-63. Rectifier and Filter Amplifier. The output of the
Converter Amplifier is applied to a rectifier circuit which
produces both a positive going and a negative going half-
wave rectified signal output (see Figure 8-20). The rectified
signals are summed to provide ac feedback for the Con-
verter Amplifier. The Filter Amplifier has a fixed gain of
approximately 6.6. The feedback circuitry of the Filter
Amplifier provides one pole of filtering. The output of the
Filter Amplifier is applied to a one pole RC filter network
for FAST ACV operation and a two pole RC filter network
for ACV operation.
FILTER
AMPLIFIER:
$495-B-4530
Figure 8-20. Simplified Average Responding AC Converter.
8-12