AD637

Rev. K | Page 12 of 20

AC MEASUREMENT ACCURACY AND CREST

FACTOR

Crest factor is often overlooked in determining the accuracy of

an ac measurement. Crest factor is defined as the ratio of the peak

Most common waveforms, such as sine and triangle waves, have

relatively low crest factors (≤2). Waveforms that resemble low

duty cycle pulse trains, such as those occurring in switching

power supplies and SCR circuits, have high crest factors. For

example, a rectangular pulse train with a 1% duty cycle has

0

Vp

T

T

CF = 1/ η

100µs

η = DUTY CYCLE =

100µs

Figure 16. Duty Cycle Timing

PULSE WIDTH (µs)

10

1

0.01

1

1000

10

100

0.1

CF = 10

CF = 3

Figure 17. AD637 Error vs. Pulse Width Rectangular Pulse

Figure 18 is a curve of additional reading error for the AD637

for a 1 V rms input signal with crest factors from 1 to 11.

A rectangular pulse train (pulse width 100 μs) is used for this

test because it is the worst-case waveform for rms measurement

(all the energy is contained in the peaks). The duty cycle and

peak amplitude were varied to produce crest factors from l to 10

while maintaining a constant 1 V rms input amplitude.

CREST FACTOR

1.5

0

–1.5

1

11

2

37

89

10

1.0

0.5

–0.5

–1.0

456

POSITIVE INPUT PULSE

Figure 18. Additional Error vs. Crest Factor

2.0

1.8

0

2.0

0.5

1.0

1.5

1.2

0.6

0.4

0.2

1.6

1.4

0.8

1.0

CF = 10

CF = 7

CF = 3

0

Figure 19. Error vs. RMS Input Level for Three Common Crest Factors

CONNECTION FOR dB OUTPUT

Another feature of the AD637 is the logarithmic, or decibel,

output. The internal circuit that computes dB works well over

a 60 dB range. Figure 20 shows the dB measurement connection.

The user selects the 0 dB level by setting R1 for the proper 0 dB

reference current, which is set to cancel the log output current

from the squarer/divider circuit at the desired 0 dB point. The

external op amp is used to provide a more convenient scale and to

allow compensation of the +0.33%/°C temperature drift of the

dB circuit. The temperature resistor R3, as shown in Figure 20,

is available from Precision Resistor Co., Inc., in Largo, Fla.

(Model PT146). Consult its website for additional information.