AD7719 datasheet(32/40 Pages) AD

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AD7719

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The absolute input voltage range on the auxiliary ADC and the

main ADC in unbuffered mode includes the range between

AGND – 30 mV to AVDD + 30 mV as a result of being unbuf-

fered. The negative absolute input voltage limit does allow

the possibility of monitoring small true bipolar signals with

respect to AGND.

Programmable Gain Amplifier

The output from the buffer on the main ADC is applied to the

input of the on-chip programmable gain amplifier (PGA). The

PGA can be programmed through eight different unipolar and

bipolar ranges. The PGA gain range is programmed via the

range bits in the ADC0CON register. With an external 2.5 V

reference applied the unipolar ranges are 0 mV to 20 mV, 0 to

40 mV, 0 mV to 80 mV, 0 mV to 160 mV, 0 mV to 320 mV,

0 mV to 640 mV, 0 V to 1.28 V and 0 to 2.56 V while bipolar

ranges are

± 20 mV, ± 40 mV, ± 80 mV, ± 160 mV, ±320 mV,

± 640 mV, ± 1.28 V and ± 2.56 V. These are the ranges that

should appear at the input to the on-chip PGA. The ADC range

matching specification of 2

µV (typ) across all ranges means that

calibration need only be carried out on a single range and does

not have to be repeated when the PGA range is changed. This is

a significant advantage when compared with similar ADCs

available on the market.

Typical matching across ranges is shown in Figure 18. Here, the

primary ADC is configured in fully-differential, bipolar mode

with an external 2.5 V reference, while an analog input voltage

of just greater than 19 mV is forced on its analog inputs. The

ADC continuously converts the DC voltage at an update rate of

5.35 Hz, i.e., SF = FF Hex. 800 conversion results in total are

gathered. The first 100 results gathered with the primary ADC

operating in the

±20 mV. The ADC range is then switched to

±40 mV and 100 more results are gathered, and so on until the

last 100 samples are gathered with the ADC configured in the

±2.5 V range. From Figure 18, the variation in the sample mean

through each range, i.e., the range matching, is seen to be on

the order of 2

µV.

The auxiliary ADC does not incorporate an eight range PGA.

The aux ADC operates at a gain of 1 or a gain of 2 as deter-

mined by the ARN bit in the AD1CON register.

100

200

300

400

500

600

700

800

SAMPLE COUNT

19.372

19.371

19.370

19.369

19.368

19.367

19.366

19.365

19.364

ADC RANGE

Figure 18. Main ADC Range Matching

Bipolar/Unipolar Configuration

The analog inputs on the AD7719 can accept either unipolar or

bipolar input voltage ranges. Bipolar input ranges does not imply

that the part can handle negative voltages with respect to system

AGND. Unipolar and bipolar signals on the AIN(+) input on

the main ADC are referenced to the voltage on the respective

AIN(–) input. AIN(+) and AIN(–) refer to the signals seen by

the modulator that come from the output of the multiplexer as

shown in Figures 16 and 17.

For example, if AIN(–) is 2.5 V and the main ADC is config-

ured for an analog input range of 0 mV to 20 mV, the input

voltage range on the AIN(+) input is 2.5 V to 2.52 V. If AIN(–)

is 2.5 V and the AD7719 is configured for an analog input range

±1.28 V, the analog input range on the AIN(+) input is 1.22 V

to 3.78 V (i.e., 2.5 V

± 1.28 V). Bipolar or unipolar options are

chosen by programming the main and Auxiliary U/

B bit in the

ADC0CON and ADC1CON registers respectively. This pro-

grams the relevant ADC for either unipolar or bipolar operation.

Programming for either unipolar or bipolar operation does not

change any of the input signal conditioning; it simply changes the

data output coding and the points on the transfer function

where calibrations occur.

Table XVIII. Max Resistance for No 16-Bit Gain Error (Unbuffered Mode)

External Capacitance

Gain

0 pF

50 pF

100 pF

500 pF

1000 pF

5000 pF

111.3K

27.8K

16.7K

4.5K

2.58K

700

53.7K

13.5K

8.1K

2.2K

1.26K

360

25.4K

6.4K

3.9K

1.0K

600

170

8–128

10.7K

2.9K

1.7K

480

270

Table XIX. Max Resistance for No 20-Bit Gain Error (Unbuffered Mode)

External Capacitance

Gain

0 pF

50 pF

100 pF

500 pF

1000 pF

5000 pF

84.9K

21.1K

12.5K

3.2K

1.77K

440

42.0K

10.4K

6.1K

1.6K

880

220

20.5K

5.0K

2.9K

790K

430

110

8–128

8.8K

2.3K

1.3K

370

195

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