SP8531 データシートの表示（PDF） - Signal Processing Technologies

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SP8531

12-Bit Sampling Serial Out Analog to Digital Converter

Signal Processing Technologies 

VIN

CLOCK IN

DATA OUT

STATUS OUT

CHIP SELECT

SHUTDOWN

1 N.C.

GAIN ADJUST 16

2 N.C.

REF OUT 15

3 VIN

4 AGND

N.C. 14

OFFSET ADJ. 13

5 VSS

SP8531

6 SCLK

VDA 12

VDD 11

7 DOUT

SD 10

8 STATUS

CS 9

10kOhms

5kOhms

+5V

0.1µF

6.8µF 0.1µF

2kOhms

+

6.8µF

* Optional filter capacitor is helpful in a noisy pc board application.

Figure 1. Operating Circuit

Input Impedance

The input of the SP8531 can be modeled as a

resistor in series with a ground referenced DC

voltage source of 1.0 volt. Note that the input

resistor is a switched capacitor resistor and so its

value is inversely proportional to conversion

rate. When the ADC is in free running mode

with a 4 MHz clock applied (a conversion rate of

235 Ksps) the input resistance is nominally

600K. At a conversion rate of 117.5 Ksps the

input resistor value would double to 1.2

megohms. In order to avoid introducing an un-

adjusted gain error greater than 1 lsb, the device

must be driven by a source whose resistance is

4096 times smaller than its input impedance. At

the 4 MHz clock rate this would require a source

whose resistance was less than 146 Ohms.

Layout Considerations

Because of the high resolution and linearity of

the SP8531, system design considerations such

as ground path impedance and contact

resistance become very important.

To avoid introducing distortion when driving

the analog inputs of these devices, the source

resistance must be very low, or constant with

signal level. Note that in the operating circuit

there is no connection made between VDA

(Pin 12) and the system power supply. This is

because the analog supply pin (VDA) is

connected internally to the digital supply pin

(VDD) through a ten ohm resistor.

This ten ohm resistor when combined with a

parallel combination of 6.8µF tantalum and

0.1µF ceramic capacitor between VDA and

analog ground, will provide some immunity to

noise which resides on the system supply. To

maintain maximum system accuracy, the

supply connected to the VDD pin should be well

isolated from digital supplies and wide load

variations.

To limit effects of digital switching elsewhere

in a system, it often makes sense to run a

separate +5V supply conductor from the supply

SP8531DS/01

SP8531 12-Bit Sampling Serial Out Analog to Digital Converter

6

© Copyright 1999 Sipex Corporation

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