AD637JRZ Analog Devices Inc, AD637JRZ Datasheet - Page 12

• Manufacturer Part Number: AD637JRZ
• Description: IC RMS/DC CONV PREC WB 16-SOIC
• Manufacturer: Analog Devices Inc

Specifications of AD637JRZ

• Current - Supply: 2.2mA
• Voltage - Supply: ±3.0V ~ 18V
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.300", 7.5mm Width)
• Accuracy %: 0.25%
• Bandwidth: 200kHz
• Supply Current: 2.2mA
• Power Dissipation Pd: 108mW
• Supply Voltage Range: ± 3V To ± 18V
• Digital Ic Case Style: SOIC
• No. Of Pins: 16
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: AD637-EVALZ - BOARD EVALUATION FOR AD637
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Lead free / RoHS Compliant

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
signal amplitude to the rms value of the signal (CF = V
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
a crest factor of 10 (CF = 1 η ).
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.
AD637
0.01
0.1
10
1
Figure 17. AD637 Error vs. Pulse Width Rectangular Pulse
1
0
Vp
100µs
Figure 16. Duty Cycle Timing
T
e
0
10
PULSE WIDTH (µs)
η = DUTY CYCLE =
CF = 1/
e
IN
(RMS) = 1 V RMS
η
100
C
AV
100µs
CF = 10
CF = 3
= 22µF
T
P
/V rms).
1000
Rev. K | Page 12 of 20
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.
Figure 19. Error vs. RMS Input Level for Three Common Crest Factors
–0.5
–1.0
–1.5
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
1.5
1.0
0.5
0
0
0
1
POSITIVE INPUT PULSE
2
Figure 18. Additional Error vs. Crest Factor
C
AV
3
0.5
= 22µF
CF = 10
CF = 7
4
CREST FACTOR
5
V
IN
(V RMS)
1.0
6
CF = 3
7
8
1.5
9
10
2.0
11