AD7888

–15–

AD7888 to MC68HC11

The Serial Peripheral Interface (SPI) on the MC68HC11 is

configured for Master Mode (MSTR = 1), Clock Polarity Bit

(CPOL) = 1 and the Clock Phase Bit (CPHA) = 1. The SPI is

configured by writing to the SPI Control Register (SPCR)—see

68HC11 User Manual. The serial transfer will take place as two

8-bit operations. A connection diagram is shown in Figure 20.

DOUT

DIN

SCLK

CS

*ADDITIONAL PINS OMITTED FOR CLARITY

MC68HC11*

AD7888*

SCLK/PD4

MISO/PD2

MOSI/PD3

PA0

Figure 20. Interfacing to the MC68HC11

AD7888 to 8051

It is possible to implement a serial interface using the data

ports on the 8051. This allows a full duplex serial transfer to be

implemented. The technique involves “bit-banging” an I/O port

(e.g., P1.0) to generate a serial clock and using two other I/O

ports (e.g., P1.1 and P1.2) to shift data in and out—see

Figure 21.

DOUT

DIN

SCLK

CS

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7888*

8051*

P1.3

P1.0

P1.1

P1.2

Figure 21. Interfacing to the 8051 Using I/O Ports

AD7888 to PIC16C6x/7x

The PIC16C6x Synchronous Serial Port (SSP) is configured

as an SPI Master with the Clock Polarity Bit = 1. This is done

by writing to the Synchronous Serial Port Control Register

(SSPCON). See user PIC16/17 Microcontroller User Manual.

Figure 22 shows the hardware connections needed to interface

to the PIC16/17. In this example I/O port RA1 is being used to

pulse

CS. This microcontroller only transfers eight bits of data

during each serial transfer operation. Therefore, two consecutive

read/write operations are needed.

DOUT

DIN

SCLK

CS

*ADDITIONAL PINS OMITTED FOR CLARITY

AD7888*

PIC16C6x / 7x*

SCK/RC3

SDO/RC5

SDI/RC4

RA1

Figure 22. Interfacing to the PIC16C6x/17x

APPLICATION HINTS

Grounding and Layout

The AD7888 has very good immunity to noise on the power

supplies as can be seen by the PSRR vs. Frequency graph.

However, care should still be taken with regard to grounding

and layout.

The printed circuit board that houses the AD7888 should be

designed so the analog and digital sections are separated and

confined to certain areas of the board. This facilitates the use of

ground planes that can be easily separated. A minimum etch

technique is generally best for ground planes as it gives the best

shielding. Digital and analog ground planes should be joined in

only one place. Both AGND pins of the AD7888 should be

sunk in the AGND plane. The AGND plane and DGND plane

connection should be made at one point only, a star ground

point that should be established as close as possible to an AGND

pin of the AD7888.

Avoid running digital lines under the device as these will couple

noise onto the die. The analog ground plane should be allowed

to run under the AD7888 to avoid noise coupling. The power

supply lines to the AD7888 should use as large a trace as pos-

sible to provide low impedance paths and reduce the effects of

glitches on the power supply line. Fast switching signals like

clocks should be shielded with digital ground to avoid radiating

noise to other sections of the board, and clock signals should

never be run near the analog inputs. Avoid crossover of digital

and analog signals. Traces on opposite sides of the board should

run at right angles to each other. This will reduce the effects of

feedthrough through the board. A microstrip technique is by far

the best but is not always possible with a double-sided board. In

this technique, the component side of the board is dedicated to

ground planes while signals are placed on the solder side.

Good decoupling is also important. All analog supplies should

be decoupled with 10

µF tantalum in parallel with 0.1 µF capaci-

tors to AGND. To achieve the best from these decoupling

components, they must be placed as close as possible to the

device, ideally right up against the device.

Evaluating the AD7888 Performance

The recommended layout for the AD7888 is outlined in the

evaluation board for the AD7888. The evaluation board pack-

age includes a fully assembled and tested evaluation board,

documentation, and software for controlling the board from the

PC via the EVAL-CONTROL BOARD. The EVAL-CONTROL

BOARD can be used in conjunction with the AD7888 evalua-

tion board, as well as many other Analog Devices evaluation

boards ending in the CB designator, to demonstrate/evaluate

the ac and dc performance of the AD7888.

The software allows the user to perform ac (fast Fourier trans-

form) and dc (histogram of codes) tests on the AD7888.

REV. C