LMV321IDBVT datasheet(19/50 Pages) TI1

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R E F

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LMV358, LMV321, LMV324, LMV324S

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SLOS263W – AUGUST 1999 – REVISED OCTOBER 2014

Typical Application (continued)

9.1.1 Design Requirements

The design requirements are as follows:

•

Supply voltage: 2.7 V

•

Reference voltage: 2.5 V

•

Input: 0.5 to 2 V

•

Output differential: ±1.5 V

9.1.2 Detailed Design Procedure

The circuit in Figure 46 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)

(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 will be one half of VREF (see Equation 7).

(3)

VOUT+ = VIN

(4)

VOUT– = VREF – VIN

(5)

VDIFF = 2×VIN – VREF

(6)

(7)

9.1.2.1 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. Because LMV358 has a bandwidth of 1 MHz, this circuit will only be

able to process signals with frequencies of less than 1 MHz.

9.1.2.2 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

36 k

Ω with tolerances measured to be within 2%. If the noise of the system is a key parameter, the user can

select smaller resistance values (6 k

Ω or lower) to keep the overall system noise low. This ensures that the noise

from the resistors is lower than the amplifier noise.

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