Datasheet AD8429 (Analog Devices) - 18

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AD8429 Data Sheet +VS
at the amplifier input. To calculate the noise referred to the amplifier output (RTO), simply multiply the RTI noise by the
0.1µF 10µF
gain of the instrumentation amplifier.
CC 1nF Source Resistance Noise L* R +IN 33Ω
Any sensor connected to the AD8429 has some output resistance.
C OUT D AD8429
There may also be resistance placed in series with inputs for pro-
10nF L* R
tection from either overvoltage or radio frequency interference.
REF 33Ω –IN
This combined resistance is labeled R1 and R2 in Figure 54. Any
CC
resistor, no matter how well made, has an intrinsic level of noise.
1nF
This noise is proportional to the square root of the resistor value.
0.1µF 10µF
At room temperature, the value is approximately equal to
–VS
49 4 nV/√Hz × √(resistor value in kΩ). 0 0- 973
*CHIP FERRITE BEAD.
0 For example, assuming that the combined sensor and protec- Figure 53. RFI Suppression tion resistance on the positive input is 4 kΩ, and on the negative The filter limits the input signal bandwidth, according to the input is 1 kΩ, the total noise from the input resistance is following relationship:  2 2 4  4   4  1  64  16  8.9 nV/√Hz 1 uency FilterFreq  DIFF 2πR 2 ( C  C ) D C
Voltage Noise of the Instrumentation Amplifier
1 The voltage noise of the instrumentation amplifier is calculated FilterFrequency  CM 2πRCC using three parameters: the device input noise, output noise, and the R where C G resistor noise. It is calculated as follows: D  10 CC. Total Voltage Noise = CD affects the difference signal, and CC affects the common-mode signal. Choose values of R and C OutputNoise G2 /  InputNoise2  Noise of R Resistor G  C that minimize RFI. A mismatch 2 between R × CC at the positive input and R × CC at the negative input degrades the CMRR of the AD8429. By using a value of For example, for a gain of 100, the gain resistor is 60.4 Ω. There- CD that is one magnitude larger than CC, the effect of the fore, the voltage noise of the in-amp is mismatch is reduced, and performance is improved. 45/1002 1  4 0.0604 2 2 = 1.5 nV/√Hz Resistors add noise; therefore, the choice of resistor and capacitor values depends on the desired tradeoff between noise, input impedance at high frequencies, and RFI immunity. The resistors
Current Noise of the Instrumentation Amplifier
used for the RFI filter can be the same as those used for input Current noise is calculated by multiplying the source resistance protection. by the current noise.
CALCULATING THE NOISE OF THE INPUT STAGE
For example, if the R1 source resistance in Figure 54 is 4 kΩ,
SENSOR
and the R2 source resistance is 1 kΩ, the total effect from the current noise is calculated as follows: 41.52  11.52  = 6.2 nV/√Hz
R1 RG AD8429 Total Noise Density Calculation
64 0 0-
R2
73 To determine the total noise of the in-amp, referred to input, 09 Figure 54. Source Resistance from Sensor and Protection Resistors combine the source resistance noise, voltage noise, and current noise contribution by the sum of squares method. The total noise of the amplifier front end depends on much more than the 1 nV/√Hz specification of this data sheet. There For example, if the R1 source resistance in Figure 54 is 4 kΩ, the are three main contributors: the source resistance, the voltage R2 source resistance is 1 kΩ, and the gain of the in-amps is 100, noise of the instrumentation amplifier, and the current noise of the total noise, referred to input, is the instrumentation amplifier. 2 2 2 8.9  1.5  6.2 = 11.0 nV/√Hz In the following calculations, noise is referred to the input (RTI). In other words, everything is calculated as if it appeared Rev. A | Page 18 of 20 Document Outline FEATURES APPLICATIONS PIN CONNECTION DIAGRAM GENERAL DESCRIPTION REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION ARCHITECTURE RG Power Dissipation REFERENCE TERMINAL INPUT VOLTAGE RANGE LAYOUT Common-Mode Rejection Ratio over Frequency Power Supplies and Grounding Reference Pin INPUT BIAS CURRENT RETURN PATH INPUT PROTECTION Input Voltages Beyond the Rails Large Differential Input Voltage at High Gain IMAX RADIO FREQUENCY INTERFERENCE (RFI) CALCULATING THE NOISE OF THE INPUT STAGE Source Resistance Noise Voltage Noise of the Instrumentation Amplifier Current Noise of the Instrumentation Amplifier Total Noise Density Calculation OUTLINE DIMENSIONS ORDERING GUIDE