AD7708 datasheet(38/44 Pages) AD

38 / 44 page

REV. 0

–38–

AD7708/AD7718

Nonchop Mode of Operation (

CHOP = 1)

Chopping is enabled and disabled using the

CHOP bit in the

mode register. Chopping is disabled by loading a 1 to the chop

bit in the mode register. With chopping disabled the available

output rates vary from 16.06 Hz (62.26 ms) to 1365.33 Hz (0.73 ms).

The range of applicable SF words is from 3 to 255. When the

chopping is disabled the channel output data rate is increased by a

factor of 3 compared to the situation when chopping is enabled

and using the same SF word. When used in multiplexed

applications, operation with chop disabled will offer the best

throughput time when cycling through all channels. The drawback

with chop disabled is that the drift performance is degraded

and calibration is required following a gain change or significant

temperature change. The output update and filter decimation

rate is again controlled by the SF word loaded to the filter

notches at integer multiples of the update rate. The output

update rate f

ADC

MOD

. The AD7708/AD7718 are targeted

at multiplexed applications and therefore operating with chop

disabled optimizes channel throughput time. One of the key

requirements in these applications is the selection of an SF word

so as to obtain the maximum filter rejection at 50 Hz and 60 Hz

while minimizing the channel throughput rate. This is achieved

with an SF word of 75 giving 57 dB rejection at 50 Hz, and

60 dB rejection at 60 Hz while offering a channel throughput

time of 55 ms. Using a higher SF word of 151, 50 Hz and

60 Hz rejection can be maximized at 60 dB with a channel

throughput rate of 110 ms. An SF word of 255 gives maximum

rejection at both 50 Hz and 60 Hz but the channel throughput

rate is restricted to 186 ms. Table XXI shows a quick comparison

of normal mode 50 Hz and 60 Hz rejection, settling time, and

update rate for a selection of SF words with chop both enabled

and disabled.

Programmable Gain Amplifier

The output from the buffer is applied to the input of the program-

mable gain amplifier (PGA). The PGA gain range is programmed

via the range bits in the ADCCON register. The PGA has eight

ranges. With an external 2.5 V reference applied, and a PGA

setting of 128, the unipolar analog input range is 0 mV to 20 mV,

while the bipolar analog input range is

±20 mV. With a PGA

setting of 1, the unipolar and bipolar input ranges are 2.56 V.

When operating with chop mode enabled (

CHOP = 0), 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 25. 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 = FFhex, 800

conversion results in total are gathered. The first 100 results are

gathered with the ADC operating with a PGA setting of 128.

19.367

100

200

400

300

19.371

19.370

19.369

19.368

19.372

500

19.366

19.365

19.364

600

700

800

SAMPLE COUNT

ADC RANGE

Figure 25. ADC Range Matching

The PGA setting is then switched to 64 and 100 more results

are gathered, and so on until the last 100 samples are gathered

with a PGA setting of 1. From Figure 25, the variation in the sample

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

of the order of 2

µV. When operating with chop mode disabled

(

CHOP = 1), new calibration data is needed (but not necessarily

a new calibration) to remove offset error when switching channels.

Bipolar/Unipolar Configuration

The analog inputs on the AD7708/AD7718 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. Signals in pseudo-differential mode

are referenced to AINCOM, while in fully differential mode they

are referenced to the negative input of the differential input. For

example, if AINCOM is 2.5 V and the AD7708/AD7718 AIN1

analog input is configured for an analog input range of 0 mV to

+20 mV, the input voltage range on the AIN1 input is 2.5 V to

2.52 V. If AINCOM is 2.5 V and the AD7708/AD7718 is con-

figured for an analog input range of

±1.28 V, the analog input

range on the AIN1 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

B bit in the ADCCON register. 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.

Data Output Coding

When the AD7718 is configured for unipolar operation, the out-

put 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 can be represented as follows:

Code = (AIN

× GAIN × 224)/(1.024 × VREF)

where

AIN is the analog input voltage,

GAIN is the PGA gain, i.e., 1 on the 2.5 V range and 128 on

the 20 mV range.

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