AD636

Data Sheet

Rev. E | Page 12 of 16

The 2-pole post filter uses an active filter stage to provide even

greater ripple reduction without substantially increasing the

settling times over a circuit with a 1-pole filter. The values of

settling times for a constant amount of ripple. Caution should

is dependent upon this value and is independent of the post

filter. For a more detailed explanation of these topics, refer to

the RMS-to-DC Conversion Application Guide, 2nd Edition.

Rx

10kΩ

1

2

3

4

5

6

7

AD636

14

13

12

11

10

9

8

ABSOLUTE

VALUE

SQUARER

DIVIDER

BUF

CURRENT

MIRROR

+

–

+

–

+

–

+

–

C2

C3

(FOR SINGLE POLE, SHORT Rx,

REMOVE C3)

C

–V

+V

10kΩ

10kΩ

NC

NC

NC

COM

NC

dB

BUF OUT

BUF IN

NC = NO CONNECT

Figure 15. 2-Pole Post Filter

FREQUENCY (Hz)

10

0.1

1

10

100

1k

10k

p-p RIPPLE

(ONE POLE)

C2 = 4.7µF

DC ERROR

(ALL FILTERS)

p-p RIPPLE

(TWO POLE)

p-p RIPPLE

(STANDARD CONNECTION)

Figure 16. Performance Features of Various Filter Types

A COMPLETE AC DIGITAL VOLTMETER

Figure 17 shows a design for a complete low power ac digital

voltmeter circuit based on the AD636. The 10 MΩ input

attenuator allows full-scale ranges of 200 mV, 2 V, 20 V, and

200 V rms. Signals are capacitively coupled to the AD636 buffer

amplifier, which is connected in an ac bootstrapped configuration

to minimize loading. The buffer then drives the 6.7 kΩ input

impedance of the AD636. The COM terminal of the ADC

provides the false ground required by the AD636 for single-

supply operation. An AD589 1.2 V reference diode is used to

provide a stable 100 mV reference for the ADC in the linear

rms mode; in the dB mode, a 1N4148 diode is inserted in series

to provide correction for the temperature coefficient of the dB

scale factor. Adjust R13 to calibrate the meter for an accurate

readout at full scale.

Calibration of the dB range is accomplished by adjusting R9

for the desired 0 dB reference point, and then adjusting R14 for the

desired dB scale factor (a scale of 10 counts per dB is convenient).

Total power supply current for this circuit is typically 2.8 mA

using a 7106-type ADC.

A LOW POWER, HIGH INPUT, IMPEDANCE dB METER

The portable dB meter circuit combines the functions of the

AD636 rms converter, the AD589 voltage reference, and a



μ A77

6 low power operational amplifier (see Figure 18). This

meter offers excellent bandwidth and superior high and low

level accuracy while consuming minimal power from a

standard 9 V transistor radio battery.

In this circuit, the built-in buffer amplifier of the AD636 is

used as a bootstrapped input stage increasing the normal 6.7 kΩ

Circuit Description

with D1 and D2, provide high input voltage protection.

The buffer’s output, Pin 6, is ac-coupled to the rms converter’s

input (Pin 1) by capacitor C2. Resistor R9 is connected between

the buffer’s output, a Class A output stage, and the negative output

swing. Resistor R1 is the amplifier’s bootstrapping resistor.

With this circuit, single-supply operation is made possible by

setting ground at a point between the positive and negative

sides of the battery. This is accomplished by sending 250 μA

from the positive battery terminal through R2, then through the

1.2 V AD589 band gap reference, and finally back to the negative

side of the battery via R10. This sets ground at 1.2 V + 3.18 V

(250 μA × 12.7 kΩ) = 4.4 V below the positive battery terminal and

5.0 V (250 μA × 20 kΩ) above the negative battery terminal.

Bypass capacitors, C3 and C5, keep both sides of the battery at a

low ac impedance to ground. The AD589 band gap reference

establishes the 1.2 V regulated reference voltage, which together

with R3 and trimming Potentiometer R4, sets the 0 dB reference

Performance Data

0 dB Reference Range = 0 dBm (770 mV) to −20 dBm (77 mV) rms

0 dBm = 1 mW in 600 Ω

Accuracy with 1 kHz sine wave and 9 V dc supply:

0 dB to −40 dBm ± 0.1 dBm

0 dBm to −50 dBm ± 0.15 dBm

+10 dBm to −50 dBm ± 0.5 dBm