ssm2212 Analog Devices, Inc., ssm2212 Datasheet - Page 8

• Manufacturer Part Number: ssm2212
• Description: Audio, Dual-matched Npn Transistor Ssm2212
• Manufacturer: Analog Devices, Inc.

SSM2212
APPLICATIONS INFORMATION
FAST LOGARITHMIC AMPLIFIER
The circuit of Figure 16 is a modification of a standard
logarithmic amplifier configuration. Running the SSM2212 at
2.5 mA per side (full-scale) allows for a fast response with a wide
dynamic range. The circuit has a 7 decade current range and a
5 decade voltage range, and it is capable of 2.5 μs settling time to
1% with a 1 V to 10 V step. The output follows the equation:
V
O
=
R
3 +
R
2
R
2
kT
V
q
10V
REF
ln
V
V
REF
IN
4kΩ
R
1
6
5
AD8512
1/2
(0V TO 10V)
7
V
IN
Figure 16. Fast Logarithmic Amplifier
330pF
4kΩ
R
S
Rev. B | Page 8 of 12
4kΩ
SSM2212
2
3
330pF
To compensate for the temperature dependence of the kT/q term, a
resistor with a positive 0.35%/°C temperature coefficient is chosen
for R
nominally −1 V/decade using the component values indicated.
AD8512
+15V
–15V
8
2
4
. The output is inverted with respect to the input and is
1
R
7.5kΩ
R
500Ω
R
3
2
2
= TEL LABS QB1E (+0.35%/°C)
V
O