LT5401 datasheet(8/16 Pages) AD

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LT5401

Rev. 0

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APPLICATIONS INFORMATION

Matching and Tolerance Specifications

(∆R/R)RE specifies the resistor ratio combinations shown

in Table 1.

Table 1. Resistor Ratio Combinations

RESISTOR RATIO

NOMINAL RATIO

(R1C + R1D)/(R1A + R1B)

(R2C + R2D)/(R2A + R2B)

(R1B + R1C + R1D)/(R1A)

(R2B + R2C + R2D)/(R2A)

(R1D)/(R1A + R1B + R1C)

0.5

(R2D)/(R2A + R2B + R2C)

0.5

(∆R/R)RE/∆T specifies the rate at which the resistor ratio

match shown in Table 1 varies over temperature. The

LT5401 specifies a maximum resistor ratio match drift

of 0.5ppm/°C, which is 20 times better than discrete

10ppm/°C resistors.

(∆R/R)RT specifies the match of the ratio formed with

R1A, R1B, R1C and R1D to the same ratio formed with

R2A, R2B, R2C and R2D. The LT5401 specifies 0.003%

ratio matching, which guarantees 96.5dB of CMRR when

the LT5401 is used to configure difference and fully dif-

ferential amplifiers. This level of performance is 133 times

better than typical 0.1% resistors might achieve.

(∆R/R)AVE specifies the average match of the ratio formed

with R1A, R1B, R1C and R1D and the ratio formed with

R2A, R2B, R2C and R2D. When using the LT5401 to

configure difference and fully differential amplifiers, this

specification guarantees 25ppm gain accuracy. This level

of performance is 80 times better than typical 0.1% resis-

tors might achieve.

(∆R/R)SS specifies the match of (R1A + R1B + R1C + R1D)

to (R2A + R2B + R2C + R2D).

∆R specifies the LT5401’s absolute resistor tolerance.

∆R/∆T specifies the rate at which the LT5401’s absolute

resistance varies over temperature.

General Considerations of LT5401 with a Fully

Differential Amplifier (FDA)

The LT5401 can be used with a variety of fully differential

amplifiers from low power to high speed. The outstanding

resistor ratio matching of LT5401 provides for very high

common mode rejection and highly accurate gain over

the specified operating temperature range.

Noise

For low noise design of an LT5401 and FDA, the noise

contribution of the LT5401 resistor values with the ampli-

fier’s voltage and current noise must be considered.

Figure 1 shows a simplified noise model for a fully dif-

ferential amplifier. Table 2 lists the LT5401 resistor values

for its three nominal gains, 0dB, 6dB and –6dB. Table 3

is a partial list of recommended differential amplifiers

for use with an LT5401. Equation 1 shows the output

noise as function of FDA noise and resistor noise. Using

Table 2 resistor values and the Table 3 noise sources,

the output noise can be calculated for an LT5401 and

FDA pair. In addition, the terms of Equation 1 provide

an intuitive guide to low noise design. For a high gain

circuit, the amplifier’s voltage noise and input resistors

noise can be the dominant noise, for low gains the ampli-

fier’s current noise can increase the output noise even if

the amplifier’s voltage noise is low. For example, for 0dB

gain the LT5401 RF and RI are 1050Ω. The ADA4938

and ADA4932 voltage and current noise are (2.8nV/√Hz,

5pA/√Hz) and (3.6nV/√Hz, 1pA/√Hz) respectively. The cal-

culated output noise of an LT5401 with an ADA4938 or

with an ADA4932 is 12.5nV/√Hz and 11.1nV/√Hz respec-

tively. So, despite the lower voltage noise that ADA4938

has, it still results in higher output noise due to its higher

current noise compared to ADA4932.

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