RATIOMETRIC VOLTAGE TRANSMITTER
FUNCTIONAL DESCRIPTION
AM417
The AM417 is an integrated low cost ratiometric voltage transmitter specially designed for bridge
input signals in automotive applications. With its integrated, ratiometrical current source the
AM417 is ideally suited for the signal conditioning of piezoresistive pressure transducers and allows
an easy temperature compensation and span adjustment of a these kinds of sensors.
The AM417 consists of 3 basic functional blocks:
1. A
Ratiometrical Current Source
for transducer excitation:
The current
I
IB
can be adjusted by the variation of the resistor
R
1
by the following relation:
I
IB
=
V
VCC
10
R
1
2. An
Instrumentation Amplifier Input Stage
with a fixed gain
G
IA
= 10 for pre–amplifiing the
bridge input signal.
3. An
Open Collector Output Stage
with the following functions:
•
Voltage Output:
As output is used a voltage amplifier which has an external PNP–open col-
lector stage
T
1
which is able to push a maximum current of
I
OUT
= 5mA. The gain
G
OUT
is ad-
justable by the external resistors
R
3
and
R
4
between
G
OUT
= 2...11:
G
OUT
=
R
3
R
3
+
R
4
The gain
G
of the complete system becomes then
G
=
G
IA
G
OUT
.
•
Current Limitation:
A simple clamp stage for the output pin
VOUT
limits the voltage drop
against
VCC
to
V
OUT
max
=
V
VCC
−
15
V
BE
(
T
1
)
.
.
The maximum current can be set by adding a resistor in series to the Emitter of the transistor
T
1
at the output stage (see figure 4). For the maximum output current is valid:
I
OUT
max
V
TRESH
−
V
BE
(
T
1
)
370mV
=
≈
.
R
2
R
2
If no current limitation is required, the Emitter of the transistor
T
1
has to be directly connected to
VCC
(R
2
= 0Ω). A proper thermic coupling of the Transistor
T
1
(V
BE
–Drift: –2mV/°C typ.)
and the AM417 reduces the resulting temperature drift of
I
OUT
and increases the performance
of the current limitation.
Adjustment of Output Voltage Range
The span of the output voltage could be adjusted by the gain
G
OUT
of the output stage. The offset of
the output voltage can be adjusted in the same way as the adjustment of the sensor offset using the
resistors
R
O1
and
R
O2
(figure 4).
analog microelectronics
April 99
4/6
AME [ ANALOG MICROELECTRONICS ]
