Agilent 34970A/34972A Data Acquisition / Switch Unit
Agilent 34970A/34972A Data Acquisition / Switch Unit
Agilent 34970A/34972A Data Acquisition / Switch Unit
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Chapter 5 Theory of Operation<br />
Internal DMM<br />
AC Circuit<br />
Unless otherwise noted, components in this discussion are located on the<br />
A4 circuit assembly (34970-66504).<br />
The multimeter uses a true RMS ac-to-dc converter to measure AC<br />
voltages and currents. The ac-to-dc converter changes the input AC<br />
voltage to a DC voltage. All voltage ranging is performed in the AC<br />
circuit so that the input to the multimeter’s DC circuitry (AC_OUT) is<br />
nominally 2 VDC for a full scale AC input. The DC amplifier is always<br />
configured for x1 gain in AC functions (voltage, current, frequency, and<br />
period). Relay K104 connects the AC circuit to either the Input HI<br />
terminal or to R121, the current function voltage sense point. Note that<br />
the input to the AC circuit may contain a DC bias from the applied AC<br />
signal.<br />
Input coupling capacitor C301 blocks the DC portion of the input signal.<br />
Only the AC component of the input signal is measured by the<br />
multimeter. The AC circuit voltage ranging comprises two gain stages<br />
U301 and U305/U312. The voltage gains for each stage are tabulated<br />
below.<br />
Function Range Shunt<br />
Resistor<br />
ACV, Freq,<br />
or Period<br />
ACI<br />
100 mV<br />
1 V<br />
10 V<br />
100 V<br />
300 V<br />
10 mA<br />
100 mA<br />
1 A<br />
5.1<br />
0.1<br />
0.1<br />
1st Stage 2nd Stage ADC Input<br />
x0.2<br />
x0.2<br />
x0.2<br />
x0.002<br />
x0.002<br />
x0.2<br />
x0.2<br />
x0.2<br />
x100<br />
x10<br />
x1<br />
x10<br />
x1<br />
x100<br />
x100<br />
x100<br />
2 VDC<br />
2 VDC<br />
2 VDC<br />
2 VDC<br />
1.4 VDC<br />
0.2 VDC<br />
1 VDC<br />
2VDC<br />
The 1st stage is a compensated attenuator implementing a gain of x0.2<br />
or x0.002 as selected by U304A and U304D. Each voltage range has a<br />
unique 50 kHz frequency response correction produced by a<br />
programmable variable capacitor connected across R304.<br />
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