AM411

Low-Cost Voltage Transmitter IC

March 2015 - Rev. 4.0

Page 7/10

www.analogmicro.de

2. Transfer Function

AM411’s transfer function is given by:

(

)

ZA

IN

IA

OP

OUT

V

V

G

G

V

+

⋅

⋅

=

with

2

1

1

R

R

G

OP

+

=

(1)

with

VOUT = output voltage

VIN = differential input voltage

VZA = voltage at pin ZA

GIA = the instrumentation amplifier’s internal gain (GIA = 5)

GOP = the output stage’s gain set by R1 and R2

R1, R2 = external resistors used to adjust GOP (see Figure 2)

3. Choosing the supply voltage

In principle AM411 can be used in the complete specified supply voltage range (VCC = 6 .. 35 V), but it has to

be noted that for small supply voltages VCC ≤ 18 V the possible maximum output voltage VOUT,MAX is given

by

2:

V

V

V

CC

MAX

OUT

5

,

−

=

(2)

For example a supply voltage of VCC = 10 V limits the possible maximum output voltage to VOUT,MAX = 5 V.

4. Adjusting the system’s gain and offset

As can be seen from equation (1) AM411’s gain can be adjusted using the external resistors R1 and R2 and a

positive offset can be set using pin ZA

3.

R1 and R2 can be calculated for a given system with a desired maximum and minimum output voltage as well

as a given maximum and minimum differential input voltage using the following equation:

(

)

1

5

min

,

max

,

min

,

max

,

2

1

−

−

⋅

−

=

IN

IN

OUT

OUT

V

V

V

V

R

R

(3)

with

VIN,min = minimum differential input voltage

VIN,max = maximum differential input voltage

VOUT,min = minimum output voltage

VOUT,max = maximum output voltage

Using VOUT,min the required voltage at pin ZA can be determined using equations (1) and (3)

(

)

min

,

min

,

min

,

max

,

min

,

max

,

5

5

IN

OUT

OUT

OUT

IN

IN

ZA

V

V

V

V

V

V

V

⋅

−

⋅

−

−

⋅

=

(4)

Please note, that for a positive differential input voltage signal, VIN ≥ 0, a mimum output voltage VOUT,min = 0 V

can only be achieved if the minimum input voltage VIN,min = 0 V and pin ZA is connected to GND. For a nega-

tive differential or bidirectional differential input voltage signals with VIN,min < 0, a voltage VZA ≥ 5 · VIN,min at pin

ZA is necessary for operation. Furthermore the boundary conditions in Table 2 have to be respected during

the dimensioning of R1 and R2.

2 For supply voltages V

CC > 18 V the maximum output voltage VOUT,max cannot be larger than 13 V.

3 Please note, that only a positive offset can be set using pin ZA.

Figure 2: AM411 with necessary external components