AD636JD datasheet(6/8 Pages) AD

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AD636

REV. B

–6–

Addition of an external resistor in parallel with RE alters this

voltage divider such that increased negative swing is possible.

Figure 11 shows the value of REXTERNAL for a particular ratio of

VPEAK to –VS for several values of RLOAD. Addition, of REXTERNAL

increases the quiescent current of the buffer amplifier by an

amount equal to REXT/–VS. Nominal buffer quiescent current

with no REXTERNAL is 30

µA at –V

S = –5 V.

REXTERNAL –

1.0

0.5

01M

10k

100k

RL = 6.7k

RL = 16.7k

RL = 50k

Figure 11. Ratio of Peak Negative Swing to –VS vs.

R EXTERNAL for Several/Load Resistances

FREQUENCY RESPONSE

The AD636 utilizes a logarithmic circuit in performing the

implicit rms computation. As with any log circuit, bandwidth is

proportional to signal level. The solid lines in the graph below

represent the frequency response of the AD636 at input levels

from 1 millivolt to 1 volt rms. The dashed lines indicate the

upper frequency limits for 1%, 10%, and

±3 dB of reading

additional error. For example, note that a 1 volt rms signal will

produce less than 1% of reading additional error up to 220 kHz.

A 10 millivolt signal can be measured with 1% of reading addi-

tional error (100

µV) up to 14 kHz.

FREQUENCY – Hz

200m

100m

10m

30m

10k

100k

100

1 VOLT rms INPUT

200mV rms INPUT

100mV rms INPUT

30mV rms INPUT

10mV rms

INPUT

1mV rms INPUT

10%

3dB

10M

Figure 12. AD636 Frequency Response

AC MEASUREMENT ACCURACY AND CREST FACTOR

Crest factor is often overlooked in determining the accuracy of

an ac measurement. Crest factor is defined as the ratio of the

peak signal amplitude to the rms value of the signal (C.F. = VP/

V rms) Most common waveforms, such as sine and triangle

waves, have relatively low crest factors (<2). Waveforms that

The current mirror also produces the output current, IOUT,

which equals 2I4. IOUT can be used directly or converted to a

voltage with R2 and buffered by A4 to provide a low impedance

voltage output. The transfer function of the AD636 thus results:

VOUT = 2 R2 I rms = VIN rms

The dB output is derived from the emitter of Q3, since the volt-

age at this point is proportional to –log VIN. Emitter follower,

Q5, buffers and level shifts this voltage, so that the dB output

voltage is zero when the externally supplied emitter current

(IREF) to Q5 approximates I3.

+VS

COM

OUT

BUF

OUT

BUFFER

BUF

10k

ONE-QUADRANT

SQUARER/

DIVIDER

CAV IOUT

ABSOLUTE VALUE/

VOLTAGE –CURRENT

CONVERTER

VIN

10k

20k

|VIN|

10 A

20 A

25k

10k

IREF

–VS

CURRENT MIRROR

Figure 9. Simplified Schematic

THE AD636 BUFFER AMPLIFIER

The buffer amplifier included in the AD636 offers the user

additional application flexibility. It is important to understand

some of the characteristics of this amplifier to obtain optimum

performance. Figure 10 shows a simplified schematic of the buffer.

Since the output of an rms-to-dc converter is always positive, it

is not necessary to use a traditional complementary Class AB

output stage. In the AD636 buffer, a Class A emitter follower is

used instead. In addition to excellent positive output voltage

swing, this configuration allows the output to swing fully down

to ground in single-supply applications without the problems

associated with most IC operational amplifiers.

CURRENT

MIRROR

BUFFER

INPUT

BUFFER

OUTPUT

+VS

40k

10k

REXTERNAL

(OPTIONAL, SEE TEXT)

–VS

RLOAD

5 A

Figure 10. AD636 Buffer Amplifier Simplified Schematic

When this amplifier is used in dual-supply applications as an

input buffer amplifier driving a load resistance referred to

ground, steps must be taken to insure an adequate negative

voltage swing. For negative outputs, current will flow from the

load resistor through the 40 k

Ω emitter resistor, setting up a

voltage divider between –VS and ground. This reduced effective

–VS, will limit the available negative output swing of the buffer.

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