AD7818ARZ Просмотр технического описания (PDF) - Analog Devices

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AD7818ARZ

Dual, Low Power CMOS, Analog Front End with DSP Microcomputer

Analog Devices 

AD7817/AD7818

CIRCUIT INFORMATION

The AD7817/AD7818 are single- and four-channel, 9 µs

conversion time, 10-bit ADCs with an on-chip temperature

sensor, reference, and serial interface logic functions on a single

chip. The ADC section consists of a conventional, successive

approximation converter based around a capacitor DAC. The

AD7817/AD7818 are capable of running on a 2.7 V to 5.5 V power

supply, and they accept an analog input range of 0 V to VREF.

The on-chip temperature sensor allows an accurate measurement

of the ambient device temperature to be made. The working

measurement range of the temperature sensor is −55°C to +125°C.

The AD7817/AD7818 require a 2.5 V reference, which can be

provided from their internal reference or from an external

reference source. The on-chip reference is selected by connecting

the REFIN pin to analog ground.

CONVERTER DETAILS

Conversion is initiated by pulsing the CONVST input. The

conversion clock for the part is internally generated; therefore,

an external clock is not required, except when reading from and

writing to the serial port. The on-chip, track-and-hold goes from

track mode to hold mode, and the conversion sequence is started

on the falling edge of the CONVST signal. At this point, the BUSY

signal goes high and low again 9 µs or 27 µs later (depending on

whether an analog input or the temperature sensor is selected)

to indicate the end of the conversion process. A microcontroller

can use this signal to determine when the result of the conversion

should be read. The track-and-hold acquisition time of the

AD7817/AD7818 is 400 ns.

A temperature measurement is made by selecting the Channel 0

of the on-chip mux and carrying out a conversion on this channel.

A conversion on Channel 0 takes 27 µs to complete. Temperature

measurement is explained in the Temperature Measurement

section.

The on-chip reference is not available, however, REFIN can be

overdriven by an external reference source (2.5 V only). The effect

of reference tolerances on temperature measurements is discussed

in the Temperature Measurement Error Due to Reference Error

section.

Tie all unused analog inputs to a voltage within the nominal

analog input range to avoid noise pickup. For minimum power

consumption, tie the unused analog inputs to AGND.

Data Sheet

TYPICAL CONNECTION DIAGRAM

Figure 8 shows a typical connection diagram for the AD7817.

The AGND and DGND are connected together at the device for

good noise suppression. The BUSY line is used to interrupt the

microcontroller at the end of the conversion process, and the

serial interface is implemented using three wires (see the AD7817

Serial Interface section for more details). An external 2.5 V

reference can be connected at the REFIN pin. If an external reference

is used, connect a 10 µF capacitor between REFIN and AGND.

For applications where power consumption is a concern, use the

automatic power-down at the end of a conversion to improve

power performance. See the Power vs. Throughput section.

SUPPLY

2.7V TO 5.5V

10µF

0.1µF

3-WIRE

SERIAL

INTERFACE

0V TO 2.5V

INPUT

VDD SCLK

AIN1

RD/WR

AIN2

DOUT

AIN3

AIN4 AD7817

DIN

OPTIONAL

EXTERNAL

REFERENCE

AD780/

REF-192

AGND

DGND

CONVST

BUSY

OTI

REFIN

CS

10µF

EXTERNAL

REFERENCE

Figure 8. Typical Connection Diagram

ANALOG INPUTS

Analog Input

Figure 9 shows an equivalent circuit of the analog input structure of

the AD7817/AD7818. The two diodes, D1 and D2, provide ESD

protection for the analog inputs. Take care to ensure that the

analog input signal never exceeds the supply rails by more than

200 mV. This causes these diodes to become forward-biased

and start conducting current into the substrate. The maximum

current these diodes can conduct without causing irreversible

damage to the part is 20 mA. The C2 capacitor in Figure 9 is

typically about 4 pF and can mostly be attributed to pin

capacitance. The R1 resistor is a lumped component made up

of the on resistance of a multiplexer and a switch. This resistor

is typically about 1 kΩ. The C1 capacitor is the ADC sampling

capacitor and has a capacitance of 3 pF.

VDD

AIN

C2

4pF

D1

C1

R1

3pF

1kΩ

VBALANCE

D2

CONVERT PHASE—SWITCH OPEN

TRACK PHASE—SWITCH CLOSED

Figure 9. Equivalent Analog Input Circuit

Rev. D | Page 12 of 20

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