AD637
OPTIONAL TRIMS FOR HIGH ACCURACY
functions of input signal frequency f, and the averaging time
constant τ (τ: 25 ms/µF of averaging capacitance). As shown in
Figure 6, the averaging error is defined as the peak value of the
ac component, ripple, plus the value of the dc error.
The AD637 includes provisions to allow the user to trim out
both output offset and scale factor errors. These trims will result
in significant reduction in the maximum total error as shown in
Figure 4. This remaining error is due to a nontrimmable input
offset in the absolute value circuit and the irreducible non-
linearity of the device.
The peak value of the ac ripple component of the averaging er-
ror is defined approximately by the relationship:
50
6.3 τf
The trimming procedure on the AD637 is as follows:
in % of reading where (t > 1/f)
l. Ground the input signal, VIN and adjust R1 to give 0 V out-
put from Pin 9. Alternatively R1 can be adjusted to give the
E
O
IDEAL
O
E
correct output with the lowest expected value of VIN
.
DC ERROR = AVERAGE OF OUTPUT–IDEAL
2. Connect the desired full scale input to VIN, using either a dc
or a calibrated ac signal, trim R3 to give the correct output at
Pin 9, i.e., 1 V dc should give l.000 V dc output. Of course, a
2 V peak-to-peak sine wave should give 0.707 V dc output.
Remaining errors are due to the nonlinearity.
AVERAGE ERROR
DOUBLE-FREQUENCY
RIPPLE
TIME
5.0
Figure 6. Typical Output Waveform for a Sinusoidal Input
AD637K MAX
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 post filtering net-
work or by increasing the value of the averaging capacitor.
2.5
INTERNAL TRIM
AD637K
EXTERNAL TRIM
0
The dc error appears as a frequency dependent offset at the
output of the AD637 and follows the equation:
1
in % of reading
0.16 + 6.4τ2 f 2
2.5
AD637K: 0.5mV ؎0.2%
0.25mV ؎0.05%
EXTERNAL
Since the averaging time constant, set by CAV, directly sets the
time that the rms converter “holds” the input signal during
computation, the magnitude of the dc error is determined only
by CAV and will not be affected by post filtering.
5.0
0
0.5
1.0
1.5
2.0
INPUT LEVEL – Volts
100
Figure 4. Max Total Error vs. Input Level AD637K
Internal and External Trims
BUFFER
AD637
10
1
14
13
12
11
10
9
R4
147⍀
ABSOLUTE
VALUE
2
3
PEAK RIPPLE
V
IN
+V
S
1.0
BIAS
SECTION
OUTPUT
OFFSET
ADJUST
SQUARER/DIVIDER
R1
50k⍀
+V
–V
4
5
S
DC ERROR
R2
1M⍀
25k⍀
S
–V
S
0.1
10
25k⍀
100
1k
10k
6
7
+
V rms
OUT
SINEWAVE INPUT FREQUENCY – Hz
C
AV
FILTER
Figure 7. Comparison of Percent DC Error to the Percent
Peak Ripple over Frequency Using the AD637 in the Stan-
dard RMS Connection with a 1 × µF CAV
8
R3
1k⍀
The ac ripple component of averaging error can be greatly
reduced by increasing the value of the averaging capacitor.
There are two major disadvantages to this: first, the value of the
averaging capacitor will become extremely large and second, the
settling time of the AD637 increases in direct proportion to the
value of the averaging capacitor (Ts = 115 ms/µF of averaging
capacitance). A preferable method of reducing the ripple is
through the use of the post filter network, shown in Figure 8.
This network can be used in either a one or two pole configura-
tion. For most applications the single pole filter will give the
best overall compromise between ripple and settling time.
SCALE FACTOR ADJUST,
؎2%
Figure 5. Optional External Gain and Offset Trims
CHOOSING THE AVERAGING TIME CONSTANT
The AD637 will compute the true rms value of both dc and ac
input signals. At dc the output will track the absolute value of
the input exactly; with ac signals the AD637’s output will ap-
proach the true rms value of the input. The deviation from the
ideal rms value is due to an averaging error. The averaging error
is comprised of an ac and dc component. Both components are
REV. E
–5–