AD637JR Просмотр технического описания (PDF) - Analog Devices

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AD637JR

High Precision, Wide-Band RMS-to-DC Converter

Analog Devices 

AD637

2.0

1.8

1.6

1.4

1.2

CF = 10

1.0

0.8

CF = 7

0.6

0.4

0.2

CF = 3

0.0

0.5

1.0

1.5

2.0

VIN – V rms

Figure 13. Error vs. RMS Input Level for Three Common

Crest Factors

the energy is contained in the peaks). The duty cycle and peak

amplitude were varied to produce crest factors from l to 10

while maintaining a constant 1 volt rms input amplitude.

CONNECTION FOR dB OUTPUT

Another feature of the AD637 is the logarithmic or decibel out-

put. The internal circuit which computes dB works well over a

60 dB range. The connection for dB measurement is shown in

Figure 14. The user selects the 0 dB level by setting R1 for the

proper 0 dB reference current (which is set to exactly cancel the

log output current from the squarer/divider circuit at the desired

0 dB point). The external op amp is used to provide a more

convenient scale and to allow compensation of the +0.33%/°C

temperature drift of the dB circuit. The special T.C. resistor R3

is available from Tel Labs in Londenderry, New Hampshire

(model Q-81) and from Precision Resistor Inc., Hillside, N.J.

(model PT146).

DB CALIBRATION

1. Set VIN = 1.00 V dc or 1.00 V rms

2. Adjust R1 for 0 dB out = 0.00 V

3. Set VIN = 0.1 V dc or 0.10 V rms

4. Adjust R2 for dB out = – 2.00 V

Any other dB reference can be used by setting VIN and R1

accordingly.

LOW FREQUENCY MEASUREMENTS

If the frequencies of the signals to be measured are below

10 Hz, the value of the averaging capacitor required to deliver

even 1% averaging error in the standard rms connection be-

comes extremely large. The circuit shown in Figure 15 shows an

alternative method of obtaining low frequency rms measure-

ments. The averaging time constant is determined by the prod-

uct of R and CAV1, in this circuit 0.5 s/µF of CAV. This circuit

permits a 20:1 reduction in the value of the averaging capacitor,

permitting the use of high quality tantalum capacitors. It is

suggested that the two pole Sallen-Key filter shown in the dia-

gram be used to obtain a low ripple level and minimize the value

of the averaging capacitor.

If the frequency of interest is below 1 Hz, or if the value of the

averaging capacitor is still too large, the 20:1 ratio can be

increased. This is accomplished by increasing the value of R. If

this is done it is suggested that a low input current, low offset

voltage amplifier like the AD548 be used instead of the internal

buffer amplifier. This is necessary to minimize the offset error

introduced by the combination of amplifier input currents and

the larger resistance.

SIGNAL

INPUT

BUFFER INPUT

1

BUFFER

AD637

NC 2

ANALOG COM

3

OUTPUT

OFFSET 4

BIAS

SECTION

CHIP

SELECT

5

DENOMINATOR

25k⍀

INPUT

6

dB

7

ABSOLUTE

VALUE

SQUARER/DIVIDER

25k⍀

FILTER

10k⍀

R1

500k⍀

+VS

+2.5 VOLTS

AD508J

BUFFER

14 OUTPUT

SIGNAL

INPUT

13

12 NC

R3

60.4⍀

1k*⍀

R2

33.2k⍀

5k⍀

+VS

dB SCALE

FACTOR

ADJUST

AD707JN

COMPENSATED

dB OUTPUT

+ 100mV/dB

–VS

11 +VS

10 –VS

RMS OUTPUT

9

+

1␮F

8

CAV

*1k⍀ + 3500ppm

TC RESISTOR TEL LAB Q81

PRECISION RESISTOR PT146

OR EQUIVALENT

0dB ADJUST

Figure 14. dB Connection

–8–

REV. E

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