Cirrus Logic AN44 User Manual

Application Note

USING THE CDBCAPTURE SYSTEM

WITH EMBEDDED A/D CONVERTERS

John Lis

AN44

INTRODUCTION

The CDBCAPTURE system can be used to collect data from embedded Analog to Digital Converters (ADCs). Thus system performance of the analog front end can be measured, analyzed and quantified. By analyzing the measured performance, noise sources can be identified, isolated, and corrective actions taken. Here CDBCAPTURE is used as an engineering tool, reducing development time during system test and integration. Another application could be using CDBCAPTURE in production for testing finished products and verifying system performance.

CDBCAPTURE

The CAPTURE interface board is a development tool that interfaces a Crystal Semiconductor ADC to a PC compatible computer. Digital data from the ADC is collected in a high speed digital FIFO, then transmitted to the PC over a serial COM port. Evaluation software is included

to analyze the data and demonstrate the ADC’s performance. The entire system consists of a CAPTURE interface board, serial cable, RS232 cable, and software.

The software included with the CDBCAPTURE system quantifies static and dynamic

performance of ADC systems. Static testing includes Histogramming and calculation of the mean, standard deviation and variance. Dynamic testing includes Fast Fourier Transforms and analysis of the power spectrum. Here Signal to Noise Ratio (SNR), Signal to Noise plus Distortion (SINAD), Signal to Distortion Ratio (SDR), and Signal to Peak Noise (SPN) figures are calculated. Time domain plots are available to visualize signals in the time domain and confirm operation.

CDBCAPTURE WITH EMBEDDED ADCs

The CDBCAPTURE circuit board is designed to directly interface with most Crystal Semiconductor ADC evaluation boards. This permits a quick and ea sy method to quantify and verify the ADC’s performance. However, it is often desirable to measure the ADC’s performance in the actual system or to measure the overall system performance. To collect data from an embedded ADC, a special serial cable needs to be designed. This cable incorporates the digital interface circuitry which exists on the evaluation board. This cable is connected to the appropriate signals on the ADC.

To collect data from an embedded ADC, the digital inte rface circuit ry on the e valuation boa rd needs to be incorporated on a modified serial

Crystal Semiconductor Corporation

P.O. Box 17847, Austin, TX 78760 (512) 445-7222 FAX 445-7581 http://www.crysta l.com

FEB ’95

AN44REV2

Using The CDBCapture System with Embedded A/D Converters

cable. Figure 1 is a block diagram illustrating the process. On the top of Figure 1, the evaluation board CDBXXXX contains digital interface circuitry which translates signals from the ADC to three standard serial signals named FRAME, SCLK and SDATA. The timing and format of these signals vary from device ty pe to device type .

In an embedded ADC application, the digital interface circuit, between the ADC and the CDBCPATURE circuit card, should be implemented on a small card attached to the serial cable as shown at the bottom of Figure 1. The appropriate digital input signals need to be identified. A method to connect to these signals from the embedded system to the cable is then

CDBXXXX EVALUATION BOARD CDBCAPTURE

5 V

A D C

INTERFACE

CIRCUIT

GND

FRAME

SCLK

SDATA

implemented. The connection scheme varies for each application and is easy to implement.

The schematic for the digital interface circuitry is provided with th e evaluation board data sheet. Locate the serial interface connector containing the signals: +5 V, GND, FRAME, SCLK, and SDATA on the schematic. Working from this connector, back to the ADC, identify the circuitry required to create the serial signals. This circuitry is implemented on a separate circuit board, attached to a 10 conductor ribbon cable with an IDC socket for its CDBCAPTURE interface. The ADC interface can be any connector scheme that is easy to implement. Possible alternatives include stake headers, test clips, circular connectors, or "D" style connectors.

SERIAL RIBBON

CABLE

EMBEDDED ADC CDBCAPTURE

A D C

2 AN44REV2

INTERFACE

CIRCUIT

Figure 1. CDBCAPTURE INTERFACE

MODIFIED

SERIAL RIBBON

CABLE

Using The CDBCapture System with Embedded A/D Converters

CS5508 EXAMPLE

The following example uses a CS5508 to illustrate the process of designing a modified serial cabl e. By examining the schematic for the CDB5508 evaluation board, the digital interface circuitry is identified. The schematic for the CDB5508 evalu ation board is pro vided in Figure 1 of the CDB5505/6/7/8 data sheet. The digital interface portion is shown in Figure 2 below. Resistors R23, R24 and R25 are not required if the

CS is always active. Also, U3B is always

CS5505 CS5506 CS5507

CS5508

DRDY

SDATA

SCLK

R23

100k

R24

100k

R25

100k

U2E

U2F

1211

1514

U3B

active when using the CAPTURE board, thus it can be changed to a general purpose buffer such as U2.

Figure 3 shows the schematic for the modified cable derived fro m Figure 2. The

DRDY, SCLK and SDATA signals are buffered to create the serial cable signals for the CAPTURE board. Five volt power is obtained from the embedded system and filtered by R1 and C1 before it is provided to the CAPTURE board. C2 is a bypass capacito r for U1.

R22

C16

10 uF

+5 DRDY

SCLK

SDATA

+5 V

GND

DRDY

SCLK

SDATA

Figure 2. CDB5508 Evaluation Board Schematic for the Digital Interface

HC4050

100k

0.1 uF

U1A

U1B

U1C

10 uF

U1D

U1E

U1F

Figure 3. Modified Serial Cable Implementation of the Digital Interface

+5 +5

DRDY

SCLK SDATA

AN44REV2 3

+ 5 hidden pages