4127
3
TYPICAL PERFORMANCE CURVES
At +25
°C with rated supplies, unless otherwise noted.
DISCUSSION OF
SPECIFICATIONS
ACCURACY
The deviation from the ideal output voltage defined as a
percent of the full scale output voltage.
INPUT/OUTPUT RANGE
The log relationships of –A log
and –A log
are
subject to the constraints specified. The 4127 can be oper-
ated with inputs lower than those given, but the accuracy
will be degraded.
FREQUENCY RESPONSE
The small-signal frequency response varies considerably
with signal level and scaling, so the frequency response is
specified under several different operating conditions.
STABILITY
The use of a monolithic transistor quad and low-drift amps
minimizes drift, but some drift remains in the scale-factor,
reference current, and input offset. Input offset consists of a
bias current plus the op amp input voltage offset divided by
the signal source resistance. Also, there is some slight drift
in conformity to the log function and in output amplifier
offset, but this is generally negligible.
THEORY OF OPERATION
The 4127 is a complete logarithmic amplifier that can be
pin-programmed to accept input currents or voltages of
either polarity. By making use of the internal current in-
verter, reference current generator, log ratio element, and
uncommitted op amp, you can generate a variety of logarith-
I
S
I
R
E
S
I
RR
10nA
Resistance (
)
RELATIONSHIP OF REFERENCE CURRENT, I
R
AND EXTERNAL RESISTOR, R
1
Reference
Current
100A
10A
1A
100nA
100k
10M
10k
100M
1M
E
O = –A log10
I
S
I
R
0
Resistance (k
)
RELATIONSHIP OF SCALE FACTOR “A”
TO GAIN SETTING RESISTOR, R
2
Scale
Factor
“A”
(V)
10
9
8
7
6
5
4
3
2
1
20
60
080
40
E
O = –A log10
|I
S|
I
R
LOG RELATIONSHIP OF
AND OUTPUT
VOLTAGE IN TERMS OF “A”
E
O = –A log10
|I
S|
I
R
0.001
IR
0.01
IR
0.1
IR
10
IR
100
IR
1000
IR
|I
S|
I
R
Input
Current
I
S
4A
2A
–2A
–4A
Output
Voltage
E
O – Volts
RELATIONSHIP OF
AND OUTPUT VOLTAGE
For I
R = 1A and A = 5V and 10V
|I
S|
I
R
0.01A
E
O = –A log
|I
S|
I
R
Input
Current
I
S
Output
Voltage
5V
10V
–5V
–10V
0.1A
1A
10A
100A
Range of
Adjustment
40dB
80dB
A = 10V
A = 5V