1995 Jul 25

9

Philips Semiconductors

Product specification

True logarithmic amplifier

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.

Fig.6 Typical application diagram.

handbook, full pagewidth

100 pF

100 pF

33 pF

56 pF

100 pF

56 pF

330

Ω

3.3 k

Ω

R g

C lf

C lf

V ref

Vout

VP

C DEC1

C DEC2

Vin

Vin

GND1

Rg

IF input

IF filter 10.7 MHz

GND2

GND4

GND5

CE

TEST

GND3

2.5 V

circuit

enable

input

n.c.

n.c.

TDA8780M

1

2

3

4

5

6

7

8

9

10

20

19

18

17

16

15

14

13

12

11

5 V

47 nF

2.2 nF

output to

ADC

MGC117