CN-0216

Circuit Note

Rev. 0 | Page 4 of 5

Figure 5. Measured Histogram for 500 Samples Showing

the Effects of Noise

COMMON VARIATIONS

Other ADCs and circuits suitable for weigh scale applications

are discussed in CN-0102 (AD7190), CN-0107 (AD7780), CN-

0108 (AD7781), CN-0118 (AD7191), CN-0119 (AD7192), and

CN-0155 (AD7195).

The AD7171 is a 16-bit sigma-delta ADC.

For a lower power consumption solution, use the ADA4051-2.

The ADA4051-2 is a dual micropower, zero-drift amplifier with

only 20 µA of supply current per amplifier.

CIRCUIT EVALUATION AND TEST

This circuit uses the EVAL-CN0216-SDPZ circuit board and

the EVAL-SDP-CB1Z system demonstration platform (SDP)

evaluation board. The two boards have 120-pin mating

connectors, allowing for the quick setup and evaluation of the

circuit’s performance. The EVAL-CN0216-SDPZ board contains

the circuit to be evaluated, as described in this note, and the

SDP evaluation board is used with the CN-0216 evaluation

software to capture the data from the EVAL-CN0216-SDPZ

circuit board.

Equipment Needed

• PC with a USB port and Windows XP or Windows Vista

(32-bit), or Windows 7 (32-bit)

• EVAL-CN0216-SDPZ circuit evaluation board

• EVAL-SDP-CB1Z SDP evaluation board

• CN0216 evaluation software

• Tedea-Huntleigh 505H-0002-F070 load cell or equivalent

• Power supply: +6 V, or +6 V “wall wart”

Getting Started

Load the evaluation software by placing the CN0216 Evaluation

Software disc in the CD drive of the PC. Using My Computer,

locate the drive that contains the evaluation software disc and

open the Readme file. Follow the instructions contained in the

Readme file for installing and using the evaluation software.

Functional Block Diagram

See Figure 1 of this circuit note for the circuit block diagram,

and the PDF file “EVAL-CN0216-SDPZ-SCH” for the circuit

schematics. This file is contained in the CN0216 Design

Support Package.

Setup

Connect the 120-pin connector on the EVAL-CN0216-SDPZ

circuit board to the connector marked “CON A” on the

EVAL-SDP-CB1Z evaluation (SDP) board. Nylon hardware

should be used to firmly secure the two boards, using the holes

provided at the ends of the 120-pin connectors. Connect the

load cell the EVAL-CN0216-SDPZ board.

With power to the supply off, connect a +6 V power supply to

the pins marked “+6 V” and “GND” on the board. If available, a

+6 V "wall wart" can be connected to the barrel jack connector

on the board and used in place of the +6 V power supply.

Connect the USB cable supplied with the SDP board to the USB

port on the PC. Note: Do not connect the USB cable to the mini

USB connector on the SDP board at this time.

Test

Apply power to the +6 V supply (or “wall wart”) connected to

EVAL-CN0216-SDPZ circuit board. Launch the evaluation

software and connect the USB cable from the PC to the USB

mini-connector on the SDP board. The software will be able to

communicate to the SDP board if the Analog Devices System

Development Platform driver is listed in the Device Manager.

Once USB communications are established, the SDP board can

now be used to send, receive, and capture serial data from the

EVAL-CN0216-SDPZ board.

Information and details regarding how to use the evaluation

software for data capture can be found in the CN0216

Evaluation Software Readme file.

Information regarding the SDP board can be found in the

SDP User Guide.

Analyzing the Data

At least 500 samples of the ADC output data should be taken.

Once the sample set is exported to a spreadsheet program, such

as Excel, the samples can be analyzed. The standard deviation of

the samples is approximately equal to the rms noise, assuming a

Gaussian noise distribution. The peak-to-peak noise is

approximately equal to the rms value multiplied by 6.6.

The peak-to-peak noise can also be taken directly from the

sample set by simply taking the difference between the largest

and smallest sample. In practice, the results obtained using this