TDA8780M Datasheet, PDF (9/16 Pages) NXP Semiconductors – True logarithmic amplifier

Philips Semiconductors
True logarithmic amplifier
Product specification
TDA8780M
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.
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.
handbook, full pagewidth
IF filter 10.7 MHz
IF input
100 pF _{Vin 1}
20 ^{Vin 100 pF}
330 Ω
GND1 ₂
GND5
19
C _{DEC1 3}
100 pF
Rg
4
18 ^{C DEC2}
33 pF
17 ^TEST
3.3 kΩ
Rg
5
CE
16
56 pF _{C lf} ^TDA8780M _{C lf}
6
15
56 pF
2.5 V
GND2
7
V _ref
8
GND4
14
Vout
13
n.c.
9
GND3 ₁₀
n.c.
12
11 ^VP
2.2 nF
MGC117
circuit
enable
input
output to
ADC
5V
47 nF
1995 Jul 25
Fig.6 Typical application diagram.
9