EVAL-AD7417 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

EVAL-AD7417

10-Bit Digital Temperature Sensor (AD7416) and Four Single-Channel ADCs (AD7417/AD7418)

Analog Devices 

AD7416/AD7417/AD7418

TYPICAL CONNECTION DIAGRAM

Figure 2 shows a typical connection diagram for the AD7417.

Using the A0, A1, and A2 pins allows the user to select from up

to eight AD7417s on the same serial bus, if desired. An external

2.5 V reference can be connected at the REFIN pin. If an exter-

nal reference is used, a 10 µF capacitor should be connected

between REFIN and GND. SDA and SCL form the 2-wire I2C

compatible interface. For applications where power consump-

tion is of concern, the automatic power-down at the end of a

conversion should be used to improve power performance. See

Operating Modes section of this data sheet.

SUPPLY

2.7V TO

5.5V

10␮F 0.1␮F

2-WIRE

SERIAL

INTERFACE

0V TO 2.5V

INPUT

OPTIONAL

EXTERNAL

REFERENCE

AD780/

REF-192

AIN1

AIN2

AIN3

AIN4

VDD SCL

SDA

CONVST

OTI

AD7417

A0

GND

A1

A2

REFIN

10␮F FOR

EXTERNAL

REFERENCE

␮C/␮P

Figure 2. Typical Connection Diagram

ANALOG INPUTS

Figure 3 shows an equivalent circuit of the analog input struc-

ture of the AD7417 and AD7418. The two diodes, D1 and D2,

provide ESD protection for the analog inputs. Care must be

taken to ensure that the analog input signal never exceeds the

supply rails by more than 200 mV. This will cause 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

capacitor C2 in Figure 3 is typically about 4 pF and can prima-

rily be attributed to pin capacitance. The resistor R1 is a lumped

component made up of the on resistance of a multiplexer and a

switch. This resistor is typically about 1 kΩ. The capacitor C1 is

the ADC sampling capacitor and has a capacitance of 3 pF.

start of the conversion phase and is powered down at the end of

the conversion. The on-chip reference is selected by connecting

the REFIN pin to analog ground. This causes SW1 (see Figure 4)

to open and the reference amplifier to power up during a conver-

sion. Therefore, the on-chip reference is not available externally.

An external 2.5 V reference can be connected to the REFIN pin.

This has the effect of shutting down the on-chip reference circuitry.

REFIN

1.2V

EXTERNAL

REFERENCE

+ DETECT

1.2V

SW1

+

26k⍀

2.5V

BUFFER

24k⍀

Figure 4. On-Chip Reference

TEMPERATURE MEASUREMENT

A common method of measuring temperature is to exploit the

negative temperature coefficient of a diode, or the base-emitter

voltage of a transistor, operated at a constant current. Unfortu-

nately, this technique requires calibration to null out the effect

of the absolute value of VBE, which varies from device to device.

The technique used in the AD7416/AD7417/AD7418 is to

measure the current change in VBE when the device is operated

at two different currents.

This is given by

∆VBE = KT / q × 1n(N )

where:

K is Boltzmann’s constant.

q is the charge on the electron (1.6 × 10-19 Coulombs).

T is the absolute temperature in Kelvins.

N is the ratio of the two currents.

VDD

I

N؋I

VDD

AIN

C2

4pF

D1

R1

C1

1k⍀

3pF

VBALANCE

D2

CONVERT PHASE – SWITCH OPEN

TRACK PHASE – SWITCH CLOSED

SENSING

TRANSISTOR

SENSING

TRANSISTOR

VOUT؉

TO ADC

VOUT؊

Figure 3. Equivalent Analog Input Circuit

ON-CHIP REFERENCE

The AD7416/AD7417/AD7418 has an on-chip 1.2 V band gap

reference that is gained up by a switched capacitor amplifier to

give an output of 2.5 V. The amplifier is only powered up at the

Figure 5. Temperature Measurement Technique

REV. G

–9–

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