AM452 – Voltage-to-current transducer IC

with a differential input

Analog Microelectronics GmbH

An der Fahrt 13, D – 55124 Mainz

February 2008 - Rev 1.2 - Page 8/21

Phone: +49 (0)6131/91 073-0

Fax:

+49 (0)6131/91 073-30

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

DETAILED DESCRIPTION OF FUNCTIONS

AM452 is a modular, monolithically integrated transducer which has been specially

developed for the conditioning of differential voltage signals. It consists of several function

blocks, the values of which are described in detail in the electrical specifications. Its various

function blocks are depicted in the block diagram (Figure 2) and described in the following.

Instrumentation amplifier (IA)

The instrumentation amplifier (IA) with an internal fixed gain of GIA = 5 acts as an input

stage for differential voltage signals of ± 400mV maximum. Thanks to the device's special

construction a high input impedance and high common mode rejection ratio (CMRR) are

achieved. The reference potential of the amplifier can be set externally using pin 13 or ZA,

with which the offset current at the output (e.g. 4mA) can be increased. It is thus possible

to compensate for the negative offset of the signal source (up to -400mV) or to correct that

of the instrumentation amplifier.

The following applies to the transfer function of the instrumentation amplifier:

ZA

IN

IA

OUTIA

V

V

G

V

+

=

with

0

>

OUTIA

V

(1)

where VIN describes the differential voltage between the two inputs VIN+ and VIN- and VZA

the voltage at pin 13 (ZA) of instrumentation amplifier IA.

Operational amplifier stage (OP1)

The operational amplifier stage (OP1) permits variable amplification of the IA output

signal. OP1 gain GGAIN can be set via external resistors R1 and R2 (see Figure 2). Protective

circuitry against overvoltage is integrated into the chip, limiting the voltage to the set value

of the reference voltage. The output voltage at OP1 can be tapped for control purposes at

pin 7 (VOP). This is calculated as:

GAIN

OUTIA

OP

G

V

V

⋅

=

with













+

=

1

2

1

R

R

G

GAIN

(2)

where

OUTIA

V

is not externally accessible but is connected internally to the OP1 input.

Zero adjust stage

The zero adjust stage enables a negative signal to be raised to a maximum of -400mV at

the instrumentation amplifier input by adding an additional voltage of VZA. A zero setting

which is practically offset free with regard to the following circuit modules can thus be

achieved, for example. The following applies:

IN

IA

OUTIA

ZA

V

G

V

V

∆

−

≤

max