AD637

Rev. K | Page 9 of 20

5.0

2.5

–5.0

0

0.5

1.0

0

–2.5

1.5

2.0

AD637K MAX

INTERNAL TRIM

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

25kΩ

DEN

INPUT

BUFF IN

BUFF

OUT

SQUARER/

DIVIDER

CS

dB OUTPUT

3 COMMON

BIAS

2NC

4

OUTPUT

OFFSET

R2

1MΩ

5

25kΩ

6

7

8

9

10

11

12

NC

13

14

NC

+

AD637

ABSOLUTE

VALUE

R4

147Ω

SCALE FACTOR TRIM

R1

50kΩ

OUTPUT

OFFSET

TRIM

4.7kΩ

R3

1kΩ

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

where

reading

of

%

in

f

1

τ

τ

6.3

50

>

DOUBLE-FREQUENCY

RIPPLE

TIME

AVERAGE ERROR

IDEAL

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

reading

of

%

in

f

2

2

4

.

6

16

.

0

1

τ

+

the time that the rms converter holds the input signal during

computation, the magnitude of the dc error is determined only

SINE WAVE INPUT FREQUENCY (Hz)

100

0.1

1.0

10

10k

1k

100

10

DC ERROR

PEAK RIPPLE

Figure 10. Comparison of Percent DC Error to the Percent Peak Ripple over

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

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.