Analog Devices AN-413 Application Notes

AN-413

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Evaluation Board for the AD7890, 12-Bit Serial, Data Acquisition System

INTRODUCTION

This application note describes the evaluation board for
the AD7890 12-bit serial data acquisition system. The
AD7890 is an eight-channel 12-bit data acquisition
system which operates from a single +5 V supply and
contains an input multiplexer, on-chip track-and-hold
amplifier, high speed 12-bit ADC, 2.5 V reference and a
high speed serial interface. The AD7890 accepts an ana­log input range of ±10 V, 0 to 4.096 V or 0 V to +2.5 V
depending on the version of the part being used. The

V+

SKT 11

CLKIN

IC7

1/2 74HC4050

5

DATA OUT

DATA IN

TFS

RFS

NC

NC

SCLK

DVDD

DGND

SKT 14

7

5

V

4

3

R1

10kΩ

2

V

9

1

8

6

DD

R2
10kΩ

DD

LK1

4

V

DD

R3

10kΩ

7

3

LK5

1/2 74HC125

89

56

1/2 74HC125

11

2

BA BA

V

12

3

IC8

DD

SKT 10

6

2

10kΩ

120pF

CONVST

C16

10

11

8

9

2

7

R4

V

DD

5

4

•

NORWOOD, MASSACHUSETTS 02062-9106

by Albert O’Grady

78L05

IN OUT

GND

IC6

6

CLKIN V

DATA OUT

DATA IN

TFS

RFS

SMODE

SCLK

CONVST

C

EXT

AGND AGND DGND

1

REFOUT/

AD7890

MUX OUT

15

IC1

part contains an on-chip control register accessible via
the serial port and allows control of channel selection,
conversion start, and power down of the part. This
flexible serial interface allows the AD7890 to connect
directly to digital signal processors (ADSP-2101,
TMS320C25, etc.) and microcontrollers (8XC51, 68HC11,
etc.). Full data on the AD7890 is available in the AD7890
data sheet available from Analog Devices and should be
consulted in conjunction with this application note when
using the evaluation board.

C4

C3

10µF

0.1µF

12

DD

24

REFIN

V

IN

V

IN

V

IN

V

IN

V

IN

V

IN

VIN2

V

IN

SHA IN

3

C15

0.1µF

23

8

22

7

21

6

20

5

19

4

18

3

17

16

1

13

14

SKT 13 SKT 12

LK3

APPLICATION NOTE

617/329-4700

•

2

V

IN

AD780

6

V

IC4

8

1

IC5

8

1

C6, C12

OUT

IC3

C7, C9

0.1µF

10

9

3
2

10

9

3
2

+

10µF

2* AD713JN

GND

4

SKT 9

EXT REF

8

V

IN

V

7

IN

6

V

IN

V

5

IN

VIN 4

V

3

IN

V

2

IN

V

1

IN

C8, C10
10µF

P
O
N
M

L

K

J

I

H
G

F

E

D
C

B
A

C5, C11

0.1µF

V+

SKT 8

SKT 7

SKT 6

SKT 5

SKT 4

SKT 3

SKT 2

SKT 1

LK4

V–

+

C14

10µF

B

LK2

C
A

12

14

13

5

7

6

12

14

13

5

7

6

IC5

MUX OUTSHA IN

Figure 1. Evaluation Board Circuit Diagram

Onboard components include an AD780, a pin program­mable +2.5 V or +3 V ultrahigh precision bandgap refer­ence, bus buffers for the serial data lines and input
buffer amplifiers to buffer the eight analog inputs. Inter­facing to this board is through a 9-way D-type connec­tor. External sockets are provided for the conversion
start input, analog inputs, clk in, mux out, sha in and
external reference input options.

OPERATING THE AD7893 EVALUATION BOARD
Power Supplies

This evaluation board has three analog power supply
inputs: AV

, AGND and VSS. These supplies are +15 V,

DD

0 V and –15 V respectively and are used to power the
onboard AD713 buffer amplifiers. The +15 V supply is
used in conjunction with the LM78L05, a 5 V linear regu­lator, to provide the V

for the AD7890 and the VIN for

DD

the AD780 voltage reference. There are two digital
power supply inputs, DV

and DGND, that are used to

DD

power the digital logic on the board. These supplies can
be provided through the D-type connector or through
the connection pins labelled on the board.

All supplies are decoupled to ground with 10 µF tanta­lum and 0.1 µF ceramic disc capacitors. The AV
V

supplies are decoupled to the AGND plane while the

SS

DV

supply is decoupled to the DGND plane.

DD

DD

and

The evaluation board uses extensive ground planing to
minimize any high frequency noise interference from
the onboard clocks or any other sources. Once again,
the ground planing for the analog section is kept sepa­rate from that for the digital section and they are joined
only at the AD7890 AGND and DGND pins.

Shorting Plug Options

There are five shorting plug options that must be set
before using the evaluation board. These are outlined
below:

LK1 This is a double link used to configure the bidirec-

tional buffer on the SCLK and

RFS inputs to the
AD7890. The position of this link also controls the
SMODE pin on the AD7890 configuring the part for
self-clocking or external clocking mode of opera­tion. In the external clocking mode both links of
LK1 must be in Position A, setting the SMODE pin
on the AD7890 to a logic high and routing the

RFS

and SCLK signals from the 9-way D-type connector
to the AD7890. In the self-clocking mode both links
on LK1 must be in Position B, configuring the
SMODE pin on the AD7890 to a logic low and rout­ing the

RFS and SCLK output signals from the

AD7890 to the 9-way D-type connector.

LK2 This option is used to select the reference source

for the AD7890 REFIN/REFOUT pin.
With this link in Position A, the on-chip 2.5 V refer-

ence is used as the reference for the AD7890. The
output impedance of this reference is 2 kΩ allow­ing this reference to be overdriven when using an
external 2.5 V reference. In Position B, the AD780

2.5 V reference is selected as the reference for the
AD7890. In Position C the reference for the part is
provided from the external socket SKT9.

LK3 This link, when in place, connects the MUX OUT to

the SHA IN. When this link is removed, the output
of the multiplexer appears at the MUX OUT pin
and also at MUX OUT (SKT12), and thus the user
can insert an antialiasing filter or signal condition­ing between the multiplexer and the ADC. The out­put range from the MUX OUT is 0 V to 2.5V and
the output impedance is 3.5 kΩ. The SHA IN is the
input to the on-chip track and hold and is a high
impedance input and accepts signals in the range
of 0 V to 2.5 V. The SHA IN pin can be driven
directly from SKT13 with LK3 removed.

LK4 This provides two options on each of the analog

input channels. Option one selects the analog
input for each channel from the external socket.
The second option facilitates the use of the grid for
signal conditioning and the signal can be linked to
the V

. The following table describes the options

IN

associated with LK4.

Table I. Options Associated with LK4

Option Function

A Grid to V
B SKT1 to V
C Grid to V
D SKT2 to V
E Grid to V
F SKT3 to V
G Grid to V
H SKT4 to V
I Grid to V
J SKT5 to V
K Grid to V
L SKT6 to V
M Grid to V
N SKT7 to V
O Grid to V
P SKT8 to V

LK5 This link in place connects

via buffer amplifier

IN1

via buffer amplifier

IN1

via buffer amplifier

IN2

via buffer amplifier

IN2

via buffer amplifier

IN3

via buffer amplifier

IN3

via buffer amplifier

IN4

via buffer amplifier

IN4

via buffer amplifier

IN5

via buffer amplifier

IN5

via buffer amplifier

IN6

via buffer amplifier

IN6

via buffer amplifier

IN7

via buffer amplifier

IN7

via buffer amplifier

IN8

via buffer amplifier

IN8

RFS and TFS together
and is useful in applications which require that
data be transmitted and received at the same time.

–2–