AD7709 datasheet(23/32 Pages) AD

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REV. A

AD7709

–23–

Programmable Gain Amplifier

The output from the buffer on the 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 Configuration Register. With an external 2.5 V reference

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

Typical matching across ranges is shown in Figure 15. Here, the

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 = FFh. A total of 800 conversion results are gathered.

The first 100 results gathered with the 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 15, the variation in the sample mean through each range,

i.e., the range matching, is seen to be on the order of 2

µV.

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 15. ADC Range Matching

Bipolar/Unipolar Configuration

The analog inputs on the AD7709 can accept either unipolar or

bipolar input voltage ranges. Bipolar input ranges do not imply that

the part can handle negative voltages with respect to system

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

the 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.

FULLY DIFFERENTIAL

AIN(+)

AIN(–)

AIN1

AIN2

AIN3

AIN4

AIN1

AIN2

AIN3

AIN4

MUX

ADC CHANNEL

Figure 16. Fully Differential Mode of Operation

AIN3

AIN4

AIN1/AINCOM

PSEUDO-DIFFERENTIAL

INPUT

AIN(–)

AIN(+)

AIN1

AIN2

AINCOM

ADC CHANNEL

MUX

AIN2/AINCOM

AIN3/AINCOM

AIN4/AINCOM

PSEUDO-DIFFERENTIAL

INPUT

Figure 17. Pseudo-Differential Mode of Operation

For example, if AIN(–) is 2.5 V and the ADC is configured 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 AD7709 is configured for an analog input range of

±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 UNI bit in the Configuration Register. This

programs the 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.

Data Output Coding

When the ADC is configured for unipolar operation, the output

coding is natural (straight) binary with a zero differential input

voltage resulting in a code of 000 . . . 000, a midscale voltage

resulting in a code of 100 . . . 000, and a full-scale input voltage

resulting in a code of 111 . . . 111. The output code for any analog

input voltage on the ADC can be represented as follows:

Code

AIN

GAIN

REF

××

()

1 024

where:

AIN is the analog input voltage.

GAIN is the PGA gain, i.e., 1 on the 2.56 V range and 128 on

the 20 mV range.

N = 16.

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