Operator`s manual

Figure 4-1. Make Voltage ACV measurements with the source ground attached to the SM2040 V,Ω - to
minimize “Common Mode” measurement problems.
4.1.3 AC Peak-to-Peak and Crest Factor (SM2042, SM2044)
Measurement of Peak-to-Peak, Crest Factor and AC Median values requires a repetitive waveform between 30 Hz
and 100 kHz. The DMM must be in AC voltage measurement mode, with the appropriate range selected. Knowing
the Peak-to-Peak value of the waveform is useful for setting the Threshold DAC (described below). This latter
function is a composite function, and may take over 10 seconds to perform.
4.1.4 AC Median Value Measurement (SM2042, SM2044)
To better understand the usage of this function, you should note that the DMM makes all AC voltage measurements
through an internal DC blocking capacitor. The voltage is thus “AC coupled” to the DMM. The measurement of
the Median value of the AC voltage is a DC measurement performed on the AC coupled input signal. This
measurement returns the mid-point between the positive and negative peak of the waveform. The Median value is
used for setting the comparator threshold level for best counter sensitivity and noise immunity. (It is difficult to
measure the frequency of a low duty cycle, low amplitude AC signal since there is DC shift at the comparator input
due to the internal AC coupling. The SM2042 and SM2044 overcome this problem by allowing you to set the
comparator threshold level). For further information on the usage of AC Median value and Peak-to-Peak
measurements, and the Threshold DAC, see the “Frequency and Timing Measurements” section below.
This function requires a repetitive signal. The DMM must be in AC voltage measurement mode, with the
appropriate range selected.
4.2 Current Measurements
The SM2040 measures AC and DC currents between 100 ηA and 2.5 A. Use the +I, 4WΩ terminals, being certain
to always leave the V,Ω + & - terminals disconnected. Use the AC/DC button to switch between AC and DC. The
AC current is an AC coupled True RMS measurement function. See figure 4-2 for connection.
The Current functions are protected with a 2.5 A, 250 V fuse. The 3.3mA and 33mA ranges utilize a 10Ω shunt,
while the 330mA and 2.5A ranges use a 0.1Ω shunt. In addition to the shunt resistors, there is some additional
parasitic resistance in the current measurement path associated with the fuse and the internal wiring.
Warning! Applying voltages > 35 V to the I+, I- inputs can cause personal injury and/or damage to your
DMM and computer! Think before applying any inputs to these terminals!
25 Signametrics