AD7280 datasheet(16/33 Pages) AD

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AD7280

Preliminary Technical Data

Rev. PrD | Page 16 of 33

CNVST

INTERNAL ADC

CONVERSIONS

PART 1

VOLT 2

VOLT 3

TEMP 6

tCONV

VOLT 1

INTERNAL ADC

CONVERSIONS

PART 2

VOLT 8

VOLT 9

TEMP 12

VOLT 7

INTERNAL ADC

CONVERSIONS

PART 3

VOLT 14

VOLT 15

TEMP 18

VOLT 13

TOTAL CONVERSION TIME

= ((tACQ + tCONV) x #conversions per part) - tACQ+ (#parts x tDELAY)

tDELAY

tACQ + tCONV

Figure 14. ADC conversions & Readback on a chain of 3 AD7280s

Suggested External Component Configurations on

Analog Inputs

As outlined in the Track-and –Hold section the acquisition time

of the AD7280 is selected by the status of bits D6 and D5 in the

CONTROL register. This provides flexibility in selecting external

components on the analog inputs. Included below are two

suggested configurations for placing external components on the

analog inputs to the AD7280.

Combined LP filter and Current Limiting Resistors

Please refer to Figure 15.

AD7280

Vin0

Vin1

Vin2

Vin3

Vin4

Vin5

Vin6

10k

Ω

100nF

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

Figure 15. External Series Resistance & Shunt Capacitance

The 10kΩ resistor in series with the inputs provides protection

to the analog inputs in the event of an over-voltage or under-

voltage on those inputs. The 100nF capacitor across the pseudo

differential inputs acts as a low pass filter in conjunction with

the 10kΩ resistor. The cut off frequency of the low pass filter is

318Hz. Using these external components the default acquisition

time of 400 ns may be used, which will allow a combined

acquisition and conversion time of 1µs.

Current Limiting Resistors

Please refer to Figure 16.

AD7280

Vin0

Vin1

Vin2

Vin3

Vin4

Vin5

Vin6

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

10k

Ω

Figure 16. External Series Resistance

The 10kΩ resistor in series with the inputs provides protection

to the analog inputs in the event of an over-voltage or under-

voltage on those inputs. Using these external components an

acquisition time of 1.6 µs should be used, which will allow a

combined acquisition and conversion time of 2.2µs.

AD7280 Datasheet(PDF) 16 Page - Analog Devices

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