AD7878

–13–

REV. A

Establish a single point analog ground (star ground) separate

from the logic system ground at Pin 22 (AGND) or as dose as

possible to the AD7878, as shown in Figure 22. Connect all

other grounds and Pin 7 (AD7878 DGND) to this single analog

ground point. Do not connect any other digital grounds to this

analog ground point. Low impedance analog and digital power

supply common returns are essential to low noise operation of

the ADC, so make the foil width for these tracks as wide as

possible. The use of ground planes minimizes impedance paths

and also guards the analog circuitry from digital noise. The

circuit layouts of Figures 25 and 26 have both analog and digital

ground planes, which are kept separated and only joined to-

gether at the AD7878 AGND pin.

NOISE

AGND (Pin 22) as short as possible to minimize input noise

coupling. In applications where this is not possible, use a

shielded cable between the source and the ADC. Reduce the

ground circuit impedance as much as possible since any poten-

tial difference in grounds between the signal source and the

ADC appears as an error voltage in series with the input signal.

Figure 22. Power Supply Grounding Practice

DATA ACQUISITION BOARD

Figure 23 shows the AD7878 in a data acquisition circuit that

will interface directly to either the ADSP-2100, TMS32010 or

the TMS32020. The corresponding printed circuit board (PCB)

layout and silkscreen are shown in Figures 24 to 26.

The only additional component required for a full data acquisi-

tion system is an antialiasing filter. There is a component grid

provided near the analog input on the PCB which may be used

for such a filter or any other conditioning circuitry. To facilitate

this option, a wire link (labelled LK1 on the PCB) is required

on the analog input track. This link connects the input signal to

either the component grid or directly to the buffer amplifier

driving the AD7878 analog input.

Microprocessor connections to the PCB can be made by either

of two ways:

1. 96-contact (3 ROW) Eurocard connector.

2. 26-contact (2 ROW) IDC connector.

The 96-contact Eurocard connector is directly compatible with

the ADSP-2100 Evaluation Board Prototype Expansion Con-

nector. The expansion connector on the ADSP-2100 has eight

decoded drip enable outputs labelled

ECE8 to ECE1. ECE6 is

used to drive the AD7878

CS input on the data acquisition

board. To avoid selecting onboard RAM sockets at the same

time, LK6 on the ADSP-2100 board must be removed. In addi-

tion, the expansion connector on the ADSP-2100 has four inter-

rupts labelled

EIRQ3 to EIRQ0. The AD7878 ALFL output

connects to

EIRQ0. The AD7878 and ADSP-2100 data lines

are aligned for left justified data transfer.

The 26-way IDC connector contains all the necessary contacts

for both the TMS32010 and TMS32020. There are two

switches on the data acquisition board that must be set to en-

able the appropriate interface configuration (see Table III). The

interface connections for the TMS32010/32020 and IDC signal

contact numbers are shown in Table IV and Figure 23. Note the

AD7878 CS input must be decoded from the address bus prior

to the AD7878 evaluation board for the TMS320XX interfaces.

are made via two BNC sockets labelled SKT1 and SKT2 on the

silkscreen. If the

CONVST input is derived from either the

microprocessor or ADC clock, the effects of clock noise cou-

pling will be reduced.

Table III. AD7878 PCB Switch Settings

SWITCH SETTING

Microprocessor

SW1

SW2

ADSP-2100

A

A

TMS32010

B

A

TMS32020

B

B

POWER SUPPLY CONNECTIONS

The PCB requires two analog supplies and one 5 V digital sup-

ply. Connections to the analog supplies are made directly to the

PCB as shown on the silk screen in Figure 24. The connections

are labelled V+ and V– and the range for both of these supplies

is 12 V to 15 V. Connection to the 5 V digital supply is made

through either of the two microprocessor connectors. The +5 V

and –5 V analog power supplies required by the AD7878 are

generated from two voltage regulators on the V+ and V– power

supply inputs (IC3 and IC4 in Figure 23).

COMPONENT LIST

IC1

AD711 Op Amp

IC2

AD7878 Analog-to-Digital

Converter

IC3

MC78L05 5 V Regulator

IC4

MC79L05 –5 V Regulator

IC5*

74HC00 Quad NAND Gate

IC6*

74HC04 Hex Inverter

IC7

74HC02 Quad NOR Gate

SW1

Single Pole Double Throw

SW2

Double Pole Double Throw

LK1

Wire Link for Analog Input

C1, C3, C5, C7, C9

10

µF Capacitors

C11, C13, C15

C2, C4, C6, C8, C10

0.1

µF Capacitors

C12, C14, C16

R1*, R2*

10 k

Ω Resistors

SKT1, SKT2

BNC Sockets

SKT3

26-Contact (2 Row) IDC

Connector

SKT4

96-Contact (3 Row) Eurocard

Connector

*Not required for ADSP-2100 Interface.