AM452 – Voltage-to-current transducer IC
with a differential input
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
G
IA
= 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:
V
OUTIA
=
G
IA
V
IN
+
V
ZA
with
V
OUTIA
>
0
(1)
where
V
IN
describes the differential voltage between the two inputs V
IN+
and V
IN-
and
V
ZA
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
G
GAIN
can be set via external resistors
R
1
and
R
2
(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:
V
OP
=
V
OUTIA
⋅
G
GAIN
with
G
GAIN
=
1
+
1
R
2
where
V
OUTIA
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 V
ZA
. A zero setting
which is practically offset free with regard to the following circuit modules can thus be
achieved, for example. The following applies:
V
ZA
≤
V
OUTIA
max
−
G
IA
∆
V
IN
R
(2)
Phone: +49 (0)6131/91 073-0
Fax:
+49 (0)6131/91 073-30
Internet:
http://www.analogmicro.de
Email: info@analogmicro.de
Analog Microelectronics GmbH
An der Fahrt 13, D – 55124 Mainz
February 2008 - Rev 1.2 - Page 8/21
AME [ ANALOG MICROELECTRONICS ]
