7.1.1 Design Requirements

The design requirements are as follows:

• Supply voltage: 15V

• Reference voltage: 12V

• Input: 2V to 10V

• Output differential: ±8V

7.1.2 Detailed Design Procedure

The circuit in Figure 7-1 takes a single-ended input signal, VIN, and generates two output signals, VOUT+ and

VOUT– using two amplifiers and a reference voltage, VREF. VOUT+ is the output of the first amplifier and is a

buffered version of the input signal, VIN (see Equation 1). VOUT– is the output of the second amplifier which

uses VREF to add an offset voltage to VIN and feedback to add inverting gain. The transfer function for VOUT– is

Equation 2.

VOUT+ = VIN

(1)

4

2

2

out

ref

in

3

4

1

1

R

R

R

V

V

1

V

R

R

R

R

-

æ

ö æ

ö

=

´

´

+

-

´

ç

÷ ç

÷

+

è

ø

è

ø

OUT

REF

4

3

4

2

1

1

2

+

IN

(2)

The differential output signal, VDIFF, is the difference between the two single-ended output signals, VOUT+ and

VOUT–. Equation 3 shows the transfer function for VDIFF. By applying the conditions that R1 = R2 and R3 = R4, the

transfer function is simplified into Equation 6. Using this configuration, the maximum input signal is equal to the

reference voltage and the maximum output of each amplifier is equal to the VREF. The differential output range is

2×VREF. Furthermore, the common-mode voltage is one half of VREF (see Equation 7).

2

4

2

D IF F

O U T

O U T

IN

R E F

1

3

4

1

R

R

R

V

V

V

V

1

V

1

R

R

R

R

+

-

æ

ö

æ

ö

æ

ö

=

-

=

´

+

-

´

+

ç

÷

ç

÷

ç

÷

+

è

ø

è

ø

è

ø

(3)

VOUT+ = VIN

(4)

VOUT– = VREF – VIN

(5)

VDIFF = 2×VIN – VREF

(6)

OUT

OUT

cm

REF

V

V

1

V

V

2

2

+

-

+

æ

ö

=

=

ç

÷

è

ø

(7)

7.1.2.1 Amplifier Selection

Linearity over the input range is key for good DC accuracy. The common-mode input range and the output swing

limitations determine the linearity. In general, an amplifier with rail-to-rail input and output swing is required.

Bandwidth is a key concern for this design. The RC4580 device has a bandwidth of 12MHz, therefore this circuit

is only be able to process signals with frequencies of less than 12MHz.

7.1.2.2 Passive Component Selection

Because the transfer function of VOUT– is heavily reliant on resistors (R1, R2, R3, and R4), use resistors with low

tolerances to maximize performance and minimize error. This design used resistors with resistance values of

36kΩ with tolerances measured to be within 2%. But, if the noise of the system is a key parameter, the user can

select smaller resistance values (6kΩ or lower) to keep the overall system noise low. This ensures that the noise

from the resistors is lower than the amplifier noise.

RC4580

SLOS412E – APRIL 2003 – REVISED NOVEMBER 2024

www.ti.com

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