Datasheet LT5401 (Analog Devices) - 8

link to page 8 link to page 8 link to page 9 link to page 9 link to page 9 link to page 9 link to page 9 link to page 9 link to page 9 LT5401
APPLICATIONS INFORMATION Matching and Tolerance Specifications General Considerations of LT5401 with a Fully
(∆R/R)
Differential Amplifier (FDA)
RE specifies the resistor ratio combinations shown in Table 1. The LT5401 can be used with a variety of fully differential
Table 1. Resistor Ratio Combinations
amplifiers from low power to high speed. The outstanding
RESISTOR RATIO NOMINAL RATIO
resistor ratio matching of LT5401 provides for very high common mode rejection and highly accurate gain over (R1C + R1D)/(R1A + R1B) 1 the specified operating temperature range. (R2C + R2D)/(R2A + R2B) 1 (R1B + R1C + R1D)/(R1A) 2
Noise
(R2B + R2C + R2D)/(R2A) 2 (R1D)/(R1A + R1B + R1C) 0.5 For low noise design of an LT5401 and FDA, the noise (R2D)/(R2A + R2B + R2C) 0.5 contribution of the LT5401 resistor values with the ampli- fier’s voltage and current noise must be considered. (∆R/R)RE/∆T specifies the rate at which the resistor ratio Figure 1 shows a simplified noise model for a fully dif- match shown in Table 1 varies over temperature. The ferential amplifier. Table 2 lists the LT5401 resistor values LT5401 specifies a maximum resistor ratio match drift for its three nominal gains, 0dB, 6dB and –6dB. Table 3 of 0.5ppm/°C, which is 20 times better than discrete is a partial list of recommended differential amplifiers 10ppm/°C resistors. for use with an LT5401. Equation 1 shows the output noise as function of FDA noise and resistor noise. Using (∆R/R)RT specifies the match of the ratio formed with Table 2 resistor values and the Table 3 noise sources, R1A, R1B, R1C and R1D to the same ratio formed with the output noise can be calculated for an LT5401 and R2A, R2B, R2C and R2D. The LT5401 specifies 0.003% FDA pair. In addition, the terms of Equation 1 provide ratio matching, which guarantees 96.5dB of CMRR when an intuitive guide to low noise design. For a high gain the LT5401 is used to configure difference and fully dif- circuit, the amplifier’s voltage noise and input resistors ferential amplifiers. This level of performance is 133 times noise can be the dominant noise, for low gains the ampli- better than typical 0.1% resistors might achieve. fier’s current noise can increase the output noise even if (∆R/R)AVE specifies the average match of the ratio formed the amplifier’s voltage noise is low. For example, for 0dB with R1A, R1B, R1C and R1D and the ratio formed with gain the LT5401 RF and RI are 1050Ω. The ADA4938 R2A, R2B, R2C and R2D. When using the LT5401 to and ADA4932 voltage and current noise are (2.8nV/√Hz, configure difference and fully differential amplifiers, this 5pA/√Hz) and (3.6nV/√Hz, 1pA/√Hz) respectively. The cal- specification guarantees 25ppm gain accuracy. This level culated output noise of an LT5401 with an ADA4938 or of performance is 80 times better than typical 0.1% resis- with an ADA4932 is 12.5nV/√Hz and 11.1nV/√Hz respec- tors might achieve. tively. So, despite the lower voltage noise that ADA4938 (∆R/R) has, it still results in higher output noise due to its higher SS specifies the match of (R1A + R1B + R1C + R1D) to (R2A + R2B + R2C + R2D). current noise compared to ADA4932. ∆R specifies the LT5401’s absolute resistor tolerance. ∆R/∆T specifies the rate at which the LT5401’s absolute resistance varies over temperature. Rev. 0 8 For more information www.analog.com Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Typical Application Related Parts