SBAS511C July   2010  – January 2018

PRODUCTION DATA.

1. Features
2. Applications
3. Description
1.     Device Images
4. Revision History
5. Device Comparison Table
6. Pin Configuration and Functions
7. Specifications
8. Detailed Description
1. 8.1 Overview
2. 8.2 Functional Block Diagrams
3. 8.3 Feature Description
4. 8.4 Device Functional Modes
1. 8.4.1 Reset and Power-Up
2. 8.4.2 Operating Modes
3. 8.4.3 Duty Cycling For Low Power
5. 8.5 Programming
1. 8.5.1 I2C Interface
2. 8.5.2 Slave Mode Operations
3. 8.5.3 Writing To and Reading From the Registers
4. 8.5.4 Data Format
6. 8.6 Register Map
9. Application and Implementation
1. 9.1 Application Information
2. 9.2 Typical Application
10. 10Power Supply Recommendations
11. 11Layout
12. 12Device and Documentation Support
13. 13Mechanical, Packaging, and Orderable Information

• DGS|10

#### 9.2.2.5 Noise and Input Impedance Considerations

If vn_res represents the input-referred rms noise from all the resistors, vn_op represents the input-referred rms noise of OPA333-Q1, and vn_ADC represents the input-referred rms noise of ADS1015-Q1, the total input-referred noise of the entire system, vN, can be approximated by Equation 11.

Equation 11. vN2 = vn_res2 + vn_op2 + vn_ADC/ (1 + R2 / R1)2

It is important to note that the ADC noise contribution, vn_ADC, is attenuated by the non-inverting gain stage.

If the gain of the noninverting gain stage is high (≥ 5), a good approximation for vn_res2 is given by Equation 12. The noise contribution from resistors R2, R4, R5, and R6 when referred to the input is smaller in comparison to R1 and R3 and can be neglected for approximation purposes.

Equation 12. vn_res2 = 4 · k · T · (R1 + R3) · Δf

where

• k = Boltzmann constant
• T = temperature (in kelvins)
• Δf = noise bandwidth

An approximation for the input impedance, RIN, of the application circuit is given by Equation 13. RIN can be modeled as a resistor in parallel with the shunt resistor, and can contribute to additional gain error.

Equation 13. RIN = R3 + R4

From Equation 12 and Equation 13, a trade-off exists between vN and RIN. If R3 increases, vn_res increases, and therefore, the total input-referred rms system noise, vN, increases. If R3 decreases, the input impedance, RIN, drops, and causes additional gain error.