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 16/21

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

Fax:

+49 (0)6131/91 073-30

Internet: http://www.analogmicro.de

Email: info@analogmicro.de

Here, GI is the overall gain of the instrumentation amplifier (IA) and the back-end operational

amplifier (OP1). ISET is the additional offset current which is set using a voltage at the SET

pin and which can raise the output current of the VI/ converter by a constant value.

1) Example 1: VIN = 0...100mV differential, IOUT = 4...20mA (3-wire)

For a measuring bridge with a signal of VIN = 0...100mV (without an offset) at the IA input

the external components should be dimensioned in such a way that output current IOUT is

4...20mA.

If the input signal offset is negligible, resistors R0, R1, R2, R3 and R4 must be determined.

With the two voltage dividers it is sufficient to calculate just one of the two resistors; the other

can be selected within the stipulations given by the boundary conditions. In this example a

value of 5V has been selected for VREF, with 10kΩ chosen for R2 and 5kΩ for R4. With a

current of 20mA the voltage should drop by a typical value of 540mV at resistor R0. The

following is accrued:

V

A

R

54

.

0

02

.

0

0

=

⋅

(13)

With reference to Equations (11) and (12) and the values given in Example 1 the following is

obtained:

(

)

)

5

(

5

2

5

8

)

10

1

(

5

1

.

0

02

.

0

3

0

0

1

Ω

+

Ω

⋅

⋅

+

⋅

Ω

+

⋅

⋅

=

k

R

k

R

V

R

k

R

V

A

)

5

(

5

2

5

004

.

0

3

0

Ω

+

Ω

⋅

⋅

=

k

R

k

R

V

A

By solving the above system of equations and taking the given defaults into account, the

following values are computed for the 3-wire, 4–20mA current interface:

R0 = 27Ω

R1 = 59.12kΩ

R2 = 10kΩ

R3 = 110.74kΩ

R4 = 5kΩ

R5 = 39Ω

RL = 0...600Ω

C1 = 2.2µF

If the offset output current is not exactly 4mA due to component tolerances and deviates from

this value, the voltage can be adjusted at pin 16 (SET) using voltage divider R3 and R4 (see

Figure 7) and the output value thus corrected (c.f.: Offset compensation using a voltage

divider at SET).