AD7816ARM datasheet(12/20 Pages) AD

12 / 20 page

REV. C

–12–

AD7816/AD7817/AD7818

1.2V

REFIN

SW1

2.5V

EXTERNAL

REFERENCE

DETECT

BUFFER

1.2V

26k

24k

Figure 9. On-Chip Reference

ADC TRANSFER FUNCTION

The output coding of the AD7816, AD7817, and AD7818 is

straight binary. The designed code transitions occur at succes-

sive integer LSB values (i.e., 1 LSB, 2 LSBs, etc.). The LSB

size is = 2.5 V/1024 = 2.44 mV. The ideal transfer characteristic

is shown in Figure 10 below.

ANALOG INPUT

1LSB

+2.5V•1LSB

1LSB=2.5/1024

2.44mV

111...111

111...110

111...000

011...111

000...010

000...001

000...000

Figure 10. ADC Transfer Function

TEMPERATURE MEASUREMENT

The on-chip temperature sensor can be accessed via multiplexer

Channel 0, i.e., by writing 0 0 0 to the channel address register.

The temperature is also the power on default selection. The

transfer characteristic of the temperature sensor is shown in

Figure 11 below. The result of the 10-bit conversion on Chan-

nel 0 can be converted to degrees centigrade by using the fol-

lowing equation.

TAMB = –103

°C + (ADC Code/4)

–55°C

125°C

192Dec

912Dec

ADC CODE

Figure 11. Temperature Sensor Transfer Characteristic

AIN

3pF

VDD

4pF

VBALANCE

CONVERT PHASE - SWITCH OPEN

TRACK PHASE - SWITCH CLOSED

Figure 7. Equivalent Analog Input Circuit

DC Acquisition Time

The ADC starts a new acquisition phase at the end of a conver-

sion and ends on the falling edge of the

CONVST signal. At the

end of a conversion a settling time is associated with the sam-

pling circuit. This settling time lasts approximately 100 ns. The

analog signal on VIN + is also being acquired during this settling

time. Therefore, the minimum acquisition time needed is

approximately 100 ns.

Figure 8 shows the equivalent charging circuit for the sampling

capacitor when the ADC is in its acquisition phase. R2 repre-

sents the source impedance of a buffer amplifier or resistive

network, R1 is an internal multiplexer resistance and C1 is the

sampling capacitor.

VIN

3pF

Figure 8. Equivalent Sampling Circuit

During the acquisition phase the sampling capacitor must be

charged to within a 1/2 LSB of its final value. The time it takes

to charge the sampling capacitor (TCHARGE) is given by the

following formula:

TCHARGE = 7.6

× (R2 + 1 kΩ) × 3 pF

For small values of source impedance, the settling time associ-

ated with the sampling circuit (100 ns) is, in effect, the acquisi-

tion time of the ADC. For example, with a source impedance (R2)

of 10

Ω the charge time for the sampling capacitor is approxi-

mately 23 ns. The charge time becomes significant for source

impedances of 1 k

Ω and greater.

AC Acquisition Time

In ac applications it is recommended to always buffer analog

input signals. The source impedance of the drive circuitry must

be kept as low as possible to minimize the acquisition time of

the ADC. Large values of source impedance will cause the THD

to degrade at high throughput rates.

ON-CHIP REFERENCE

The AD7816, AD7817, and AD7818 have an on-chip 1.2 V

band gap reference that is gained up to give an output of 2.5 V.

The on-chip reference is selected by connecting the REFIN pin

to analog ground. This causes SW1 (see Figure 9) to open and

the reference amplifier to power up during a conversion. There-

fore, 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 and reduc-

ing IDD by about 0.25 mA.

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