TDA8780M Philips Semiconductors, TDA8780M Datasheet - Page 9

• Manufacturer Part Number: TDA8780M
• Description: True logarithmic amplifier
• Manufacturer: Philips Semiconductors

Philips Semiconductors
APPLICATION INFORMATION
The circuit is typically connected as shown in Fig.6. The
single-ended 10.7 MHz input IF signal is applied
(arbitrarily) to one of the two input pins via a ceramic filter.
These inputs should not be DC coupled as this will disable
the on-chip feedback which sets the DC operating point of
the true logarithmic amplifier. The relatively high
impedance of these inputs facilitates correct termination of
the ceramic filter by an off-chip resistor.
1995 Jul 25
handbook, full pagewidth
True logarithmic amplifier
IF input
IF filter 10.7 MHz
330
2.5 V
3.3 k
Fig.6 Typical application diagram.
100 pF
100 pF
56 pF
C DEC1
GND2
GND3
GND1
V ref
n.c.
V in
R g
R g
C lf
1
2
3
4
5
6
7
8
9
10
TDA8780M
9
The low-frequency cut-off point is determined by the value
of capacitors connected to pins 6 and 15 which decouple
the overall DC feedback and the value of the input coupling
capacitors. The output is coupled to an analog-to-digital
converter thus the value of the voltage fed to the reference
voltage input is not critical. It could be useful in other
applications, where the output may be DC coupled to an
alternative analog-to-digital converter, to derive this
reference voltage from the centre of the input resistor
chain of the analog-to-digital converter.
MGC117
20
19
18
17
16
15
14
13
12
11
V out
V in
GND5
C DEC2
TEST
CE
GND4
V P
C lf
n.c.
2.2 nF
100 pF
33 pF
56 pF
47 nF
output to
ADC
circuit
enable
input
5 V
Product specification
TDA8780M