OPA2810 Datasheet, PDF (17/26 Pages) Texas Instruments – OPA2810 Dual High-Performance, Low-Power, Wide Supply Range, Rail-to-Rail Input/Output FET-Input Operational Amplifier

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8 Application and Implementation
OPA2810
SBOS789 – AUGUST 2017
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
8.1 Application Information
8.1.1 Noise Analysis and the Effect of Resistor Elements on Total Noise
The OPA2810 provides a very low input-referred broadband noise voltage density of 5.7 nV/√Hz while requiring a
low 7.2-mA quiescent supply current. To take full advantage of this low input noise, careful attention to the other
possible noise contributors is required. Figure 4 shows the operational amplifier noise analysis model with all the
noise terms included. In this model, all the noise terms are taken to be noise voltage or current density terms in
either nV/√Hz or pA/√Hz.
ENI
+
EO
RS
IBN
±
ERS
4kTR _S
RF
4kT
RG
IBI
4kTR _F
R_G
4kT 1.6E 20 J
at 290q K
Figure 4. Operational Amplifier Noise Analysis Model
The total output spot noise voltage can be computed as the square root of the squared contributing terms to the
output noise voltage. This computation adds all the contributing noise powers at the output by superposition, then
calculates the square root to get back to a spot noise voltage. Figure 4 shows the general form for this output
noise voltage using the terms shown in Equation 1.
( ) ^{( ) E}O ⁼
E_NI2 + (I_BNR_S )² + 4kTR_S
NG² +
IBIRF
2
+ 4kTR_FNG
(1)
Dividing this expression by the noise gain (NG = 1+R_F/R_G) gives the equivalent input referred spot noise voltage
at the noninverting input, see Equation 2.
E_N =
E_NI2
+
(I_BNR_S
)²
+
4kTR_S
+
æ
çè
I_BIR_F
NG
ö²
÷ø
+
4kTR_F
NG
(2)
Substituting large resistor values into Equation 2 can quickly dominate the total equivalent input referred noise. A
source impedance on the noninverting input of 2 kΩadds a Johnson voltage noise term equal to just that of the
amplifier itself (5.7 nV/√Hz).
Table 2 compares the noise contributions from the various terms when the OPA2810 is configured in a
noninverting gain of 5V/V as shown in Figure 5. Two cases have been considered where the resistor values in
case 2 are 10x the resistor values in case 1. The total output noise In case 1 is 31.3 nV/√Hz while the noise is
case 2 is 49.7 nV/√Hz. The large value resistors in case 2 dilute the benefits of selecting a low noise amplifier
like the OPA2810. To minimize total system noise it is beneficial to reduce the size of the resistor values,
however this also increases the amplifiers output load and results in a degradation of its distortion performance.
The increased loading also increases the dynamic power consumption of the amplifier. The circuit designer
should make the appropriate tradeoffs to maximize the amplifiers overall performance to match the system
requirements.
Copyright © 2017, Texas Instruments Incorporated
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