Datasheet

AD637
Rev. K | Page 9 of 20
5.0
2.5
–5.0
00.5
ERROR (mV)
1.0
0
–2.5
1.5 2.0
AD637K MAX
INTERNAL TRIM
00788-007
AD637K
EXTERNAL TRIM
AD637K: 0.5mV ± 0.2%
0.25mV ± 0.05%
EXTERNAL
INPUT LEVEL (V)
Figure 7. Maximum Total Error vs.
Input Level AD637K Internal and External Trims
1
25k
DEN
INPUT
BUFF IN
BUFF
OUT
SQUARER/
DIVIDER
–V
S
CS
dB OUTPUT
3 COMMON
BIAS
2NC
4
OUTPUT
OFFSET
R2
1M
5
25k
6
7 8
9
10
+V
S
11
12NC
13
14
NC
V
IN
V
OUT
=V
IN
2
V
IN
C
AV
+
AD637
ABSOLUTE
VALUE
00788-008
–V
S
+V
S
C
AV
R4
147
SCALE FACTOR TRIM
R1
5
0k
OUTPUT
OFFSET
TRIM
4.7k
–V
S
+V
S
+V
S
R3
1k
Figure 8. Optional External Gain and Offset Trims
CHOOSING THE AVERAGING TIME CONSTANT
The AD637 computes the true rms value of both dc and ac
input signals. At dc, the output tracks the absolute value of the
input exactly; with ac signals, the AD637 output approaches the
true rms value of the input. The deviation from the ideal rms
value is due to an averaging error. The averaging error
comprises an ac component and a dc component. Both
components are functions of input signal frequency f and the
averaging time constant τ (τ: 25 ms/μF of averaging capacitance).
Figure 9 shows that the averaging error is defined as the peak
value of the ac component (ripple) and the value of the dc error.
The peak value of the ac ripple component of the averaging
error is defined approximately by the relationship
()
fwherereadingof%in
f
1τ
τ6.3
50
>
DOUBLE-FREQUENCY
RIPPLE
E
O
TIME
AVERAGE ERROR
IDEAL
E
O
00788-009
DC ERROR = AVERAGE OF OUTPUT – IDEAL
Figure 9. Typical Output Waveform for a Sinusoidal Input
This ripple can add a significant amount of uncertainty to the
accuracy of the measurement being made. The uncertainty can
be significantly reduced through the use of a postfiltering
network or by increasing the value of the averaging capacitor.
The dc error appears as a frequency dependent offset at the
output of the AD637 and follows the relationship
readingof%in
f
22
4.616.0
1
τ+
Because the averaging time constant, set by C
AV
, directly sets
the time that the rms converter holds the input signal during
computation, the magnitude of the dc error is determined only
by C
AV
and is not affected by postfiltering.
SINE WAVE INPUT FREQUENCY (Hz)
100
0.1
1.0
10 10k
DC ERROR OR RIPPLE (% of Reading)
1k100
10
DC ERROR
PEAK RIPPLE
00788-010
Figure 10. Comparison of Percent DC Error to the Percent Peak Ripple over
Frequency Using the AD637 in the Standard RMS Connection with a 1 × μF C
AV
The ac ripple component of averaging error is greatly reduced
by increasing the value of the averaging capacitor. There are two
major disadvantages to this: the value of the averaging capacitor
becomes extremely large and the settling time of the AD637
increases in direct proportion to the value of the averaging
capacitor (T
S
= 115 ms/μF of averaging capacitance). A preferable
method of reducing the ripple is by using the postfilter network,
as shown in Figure 11. This network can be used in either a 1-
pole or 2-pole configuration. For most applications, the 1-pole
filter gives the best overall compromise between ripple and
settling time.