Agilent Technologies 66311B, D, 66309B Measuring Circuits Floating with Respect to the Main Output

Installation - 3

♦You cannot measure voltages greater than +25 Vdc with respect to the negative terminal of the main output. A situation where this could occur is illustrated by R1 in figure 3-8, which has only a 12 Vdc drop across it but is 36 Vdc + Vlead with respect to the negative terminal of the main output.

♦You cannot measure voltages less than −4.5 Vdc with respect to the negative terminal of the main output. A situation where this could occur is illustrated by R6 in figure 3-8, which has only a −2 Vdc drop across it but is −6 Vdc + Vlead with respect to the negative terminal of the main output.

♦When calculating the common mode voltage between the point that you wish to measure and the negative terminal of the main output, you must also include any voltage drop in the negative load lead. For example, in figure 3-8, if the voltage drop in the negative load lead is 2 V, you would not be able to correctly measure the 12 Vdc drop across R2. This is because when the voltage drop in the load lead is added to the voltage drops across R2 and R3, the resultant voltage is 26 Vdc, which exceeds the +25 Vdc common mode rating of the DVM.

Measuring Circuits that are Floating with Respect to the Main Output

In the example shown in figure 3-9, the common mode voltage between the DVM inputs and the minus terminal of the main output (output 1) includes an undefined floating voltage that may result in incorrect readings due to clipping by the internal DVM measurement circuits. This will occur when the −4.5 Vdc to + 25 Vdc common mode voltage range is exceeded.

The solution to this problem would be to provide a known or controlled common mode voltage by connecting a jumper wire from the floating voltage to be measured to the main output. In this example, the main output is set to 5V, the ac voltage to be measured is approximately 6 Vac (±8.5 Vpeak), and a jumper wire connects one side of the bias transformer to the + main output terminal. This stabilizes the common mode voltage and offsets it by the output voltage value (5 V). The peak common mode voltage is now:

+8.5V + 5 V = +13.5 V on the positive side, and −8.5V + 5 V = −3.5 V on the negative side;

with both voltages now being within the common mode range of the DVM.

			6 V Bias
			Transformer
			winding capacitance
Agilent 66309D			AC
Agilent 66311D
DVM INPUT		TO	6 Vac;
			8.5 Vpk
		DVM
	jumper wire		ACC
			winding capacitance
OUTPUT 1	+
	+ 5 V		stray
	−
			capacitance
	GND	GND	GND

Typically, low voltage with respect to GND due to internal bypass capacitors.

Undefined float voltage with respect to GND due to capacitive currents. Could be tens of volts ac or more.

Figure 3-9. Measuring Circuits Floating with Respect to the Main Output

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