Datasheet

Data Sheet AD536A
Rev. E | Page 9 of 16
14
13
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11
10
9
8
1
2
3
4
5
6
7
AD536A
25kΩ
ABSOLUTE
VALUE
SQUARER/
DIVIDER
CURRENT
MIRROR
00504-107
BUF
V
IN
NC
–V
S
C1, C
AV
C
AV
C2
+V
S
NC
NC
NC
dB
COM
R
L
I
OUT
+V
S
4.6V TO 18V
E
OUT
–E
+E
2.5V
OP77
AD580J
R1
500kΩ
0dB
REF
ADJUST
dB SCALE
FACTOR ADJUST
TEMPERATURE
COMPENSATED
dB OUTPUT
+100mV/dB
–V
S
+V
S
–V
S
+V
S
dB OUT
3mV/dB
1
SPECIAL TC COMPENSATION RESISTOR, +3300ppm/°C,
PRECISION RESISTOR COMPANY PART NUMBER AT 35 OR PART NUMBER ST35.
LINEAR
rms
OUTPUT
R6
24.9kΩ
R3
60.4Ω
R2
1
1kΩ
R4
33.2kΩ
R5
5kΩ
BUF OUT
BUF IN
+
0.1µF
7
4
3
6
2
Figure 7. dB Connection
FREQUENCY RESPONSE
The AD536A utilizes a logarithmic circuit in performing the
implicit rms computation. As with any log circuit, bandwidth
is proportional to signal level. The solid lines in the graph of
Figure 8 represent the frequency response of the AD536A at
input levels from 10 mV rms to 7 V rms. The dashed lines indicate
the upper frequency limits for 1%, 10%, and ±3 dB of reading
additional error. For example, note that a 1 V rms signal produces
less than 1% of reading additional error up to 120 kHz. A 10 mV
signal can be measured with 1% of reading additional error
(100 µV) up to only 5 kHz.
100k 1M 10M
1k 10k
10
1
0.1
0.01
V
OUT
(V)
1%
10%
±3dB
FREQUENCY (Hz)
7V rms INPUT
1V rms INPUT
100mV rms INPUT
10mV rms INPUT
00504-016
Figure 8. High Frequency Response
AC MEASUREMENT ACCURACY AND CREST
FACTOR
Crest factor is often overlooked when determining the accuracy
of an ac measurement. The definition of crest factor is the ratio
of the peak signal amplitude to the rms value of the signal
(CF = V
P
/V rms). 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√n).
Figure 9 illustrates a curve of reading error for the AD536A for
a 1 V rms input signal with crest factors from 1 to 11. A rectan-
gular pulse train (pulse width = 100 µs) was used for this test
because it is the worst-case waveform for rms measurement (all
of the energy is contained in the peaks). The duty cycle and
peak amplitude were varied to produce crest factors from 1 to
11 while maintaining a constant 1 V rms input amplitude.
η = DUTY CYCLE =
CF = 1/√η
ө
IN
(rms) = 1 V rms
100µs
T
ө
O
V
P
0
100µs
T
1
0
–1
–2
–3
–4
INCREASE IN ERROR (% of Reading)
1 2 3 4 5 6 7 8 9 10 11
CREST FACTOR
00504-017
Figure 9. Error vs. Crest Factor
INCREASE IN ERROR
(% OF READING)
1µs 10µs 100µs 1000µs
PULSE WIDTH (µs
)
10
1
0.1
1V rms
CF = 3
1V rms CF = 10
00504-018
Figure 10. Error vs. Pulse Width Rectangular Pulse