AD636
The current mirror also produces the output current, I
OUT
,
which equals 2I
4
. I
OUT
can be used directly or converted to a
voltage with R2 and buffered by A
4
to provide a low impedance
voltage output. The transfer function of the AD636 thus results:
V
OUT
=
2
R2 I rms
=
V
IN
rms
The dB output is derived from the emitter of Q
3
, since the volt-
age at this point is proportional to –log V
IN
. Emitter follower,
Q
5
, buffers and level shifts this voltage, so that the dB output
voltage is zero when the externally supplied emitter current
(I
REF
) to Q
5
approximates I
3
.
CURRENT MIRROR
14 +V
S
Addition of an external resistor in parallel with R
E
alters this
voltage divider such that increased negative swing is possible.
Figure 11 shows the value of R
EXTERNAL
for a particular ratio of
V
PEAK
to –V
S
for several values of R
LOAD.
Addition, of R
EXTERNAL
increases the quiescent current of the buffer amplifier by an
amount equal to R
EXT
/–V
S
. Nominal buffer quiescent current
with no R
EXTERNAL
is 30
µA
at –V
S
= –5 V.
1.0
RATIO OF V
PEAK
/V
SUPPLY
R
L
= 50k
0.5
R
L
= 16.7k
10
COM
R1
25k
ABSOLUTE VALUE/
VOLTAGE –CURRENT
CONVERTER
10 A
FS
I
3
20 A
FS
I
1
R4
20k
V
IN
1
4
8
R2
C
AV
I
OUT
I
4
10k
I
REF
BUF
IN BUFFER
7
A4
Q5
10k
9
R
L
dB
5 OUT
6 BUF
OUT
A3
Q1
Q3
R
L
= 6.7k
0
|
V
IN
|
+
R4
8k
0
1k
A1
R3
10k
8k
Q2 Q4
A2
ONE-QUADRANT
SQUARER/
DIVIDER
10k
R
EXTERNAL
–
100k
1M
Figure 11. Ratio of Peak Negative Swing to –V
S
vs.
R
EXTERNAL
for Several/Load Resistances
3 –V
S
Figure 9. Simplified Schematic
THE AD636 BUFFER AMPLIFIER
FREQUENCY RESPONSE
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.
+V
S
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.
1 VOLT rms INPUT
1%
200mV rms INPUT
100mV rms INPUT
30mV rms INPUT
10%
3dB
1
200m
100m
V
OUT
– Volts
30m
10m
CURRENT
MIRROR
BUFFER
OUTPUT
R
LOAD
5 A
BUFFER
INPUT
5 A
10k
R
E
40k
10mV rms
INPUT
1m
1mV rms INPUT
–V
S
R
EXTERNAL
(OPTIONAL, SEE TEXT)
100
1k
10k
100k
FREQUENCY – Hz
1M
10M
Figure 10. AD636 Buffer Amplifier Simplified Schematic
Figure 12. AD636 Frequency Response
AC MEASUREMENT ACCURACY AND CREST FACTOR
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 kΩ emitter resistor, setting up a
voltage divider between –V
S
and ground. This reduced effective
–V
S
, will limit the available negative output swing of the buffer.
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. = V
P
/
V rms) Most common waveforms, such as sine and triangle
waves, have relatively low crest factors (<2). Waveforms that
–6–
REV. B
AD [ ANALOG DEVICES ]
