Frequency Response

The MX536A/MX636 utilizes a logarithmic circuit in per-

forming the RMS computation of the input signal. The

bandwidth of the RMS converters is proportional to sig-

nal level. Figures 11 and 12 represent the frequency

MX536A and 1mV to 1V for the MX636, respectively.

The dashed lines indicate the upper frequency limits for

1%, 10%, and ±3dB of reading additional error.

Caution must be used when designing RMS measuring

systems so that overload does not occur. The input

clipping level for the MX636 is ±12V, and for the

of 3 has a peak input of 21V.

Application in a Low-Cost DVM

A low-cost digital voltmeter (DVM) using just two inte-

grated circuits plus supporting circuitry and LCD dis-

play is shown in Figure 13. The MAX130 is a 3 1/2 digit

integrating A/D converter with precision bandgap refer-

ence. The 10M

Ω input attenuator is AC coupled to pin

6 of the MX636 buffer amplifier. The output from the

MX636 is connected to the MAX130 to give a direct

reading to the LCD display.

True RMS-to-DC Converters

10

______________________________________________________________________________________

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

COMMON

N.C.

N.C.

N.C.

dB

N.C.

C2

MX536A

MX636

Figure 7. MX536A/MX636 with a One-Pole Output Filter

14

13

12

11

10

9

8

1

2

3

4

5

6

7

ABSOLUTE

VALUE

SQUARER

DIVIDER

CURRENT

MIRROR

BUF

C3

C2

* MX536A = 25k

Ω

MX636 = 10k

Ω

MX536A

MX636

Figure 8. MX536A/MX636 with a Two-Pole Output Filter

0.1

10

1k

10k

1

10

FREQUENCY (Hz)

100

PK-PK RIPPLE

PK-PK RIPPLE

(ONE POLE)

C2 = 4.7

µF

PK-PK RIPPLE

(TWO POLE)

C2 = C3 = 4.7

µF

DC ERROR

(ALL FILTERS)

MX536A

2.2

µF

1

µF

2.2

µF

2.2

µF

1

µF

MX636

4.7

µF

1

µF

4.7

µF

4.7

µF

1

µF

ONE-POLE FILTER

C2

TWO-POLE FILTER

C2

C3

Figure 9. Performance Features of Various Filter Types for

MX536A/MX636