AD7788 Просмотр технического описания (PDF) - Analog Devices

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AD7788

Low Power, 16-/24-Bit Sigma-Delta ADC

Analog Devices 

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7788/AD7789 has one differential analog input channel

that is connected to the modulator, i.e., the input is unbuffered.

Note that this unbuffered input path provides a dynamic load to

the driving source. Therefore, resistor/capacitor combinations

on the input pins can cause dc gain errors, depending on the

output impedance of the source that is driving the ADC input.

Table 13 shows the allowable external resistance/capacitance

values such that no gain error at the 16-bit level is introduced

(AD7788), while Table 14 shows the allowable external resis-

tance/capacitance values such that no gain error at the 20-bit

level is introduced (AD7789).

Table 13. External R-C Combination for No 16-Bit Gain

Error (AD7788)

C (pF)

R (Ω)

50

22.8K

100

13.1K

500

3.3K

1000

1.8K

5000

360

Table 14. External R-C Combination for No 20-Bit Gain

Error (AD7789)

C (pF)

R (Ω)

50

16.7K

100

9.6K

500

2.2K

1000

1.1K

5000

160

The absolute input voltage includes the range between GND –

30 mV and VDD + 30 mV. The negative absolute input voltage

limit does allow the possibility of monitoring small true bipolar

signals with respect to GND.

The absolute input voltage includes the range between GND –

30 mV and VDD + 30 mV. The negative absolute input voltage

limit does allow the possibility of monitoring small true bipolar

signals with respect to GND.

BIPOLAR/UNIPOLAR CONFIGURATION

The analog input to the AD7788/AD7789 can accept either

unipolar or bipolar input voltage ranges. A bipolar input range

does not imply that the part can tolerate large negative voltages

with respect to system GND. Unipolar and bipolar signals on

the AIN(+) input are referenced to the voltage on the AIN(–)

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

ured for unipolar mode, the input voltage range on the AIN(+)

pin is 2.5 V to 5 V. If the ADC is configured for bipolar mode,

AD7788/AD7789

the analog input range on the AIN(+) input is 0 V to 5 V. The

bipolar/unipolar option is chosen by programming the B/U bit

in the mode register.

DATA OUTPUT CODING

When the ADC is configured for unipolar operation, the output

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

voltage resulting in a code of 00...00, a midscale voltage result-

ing 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

Code = 2N × (AIN/VREF)

When the ADC is configured for bipolar operation, the output

code is offset binary with a negative full-scale voltage resulting

in a code of 000...000, a zero differential input voltage resulting

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

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

input voltage can be represented as

Code = 2N – 1 × [(AIN/VREF) + 1]

where AIN is the analog input voltage and N = 16 for the

AD7788 and 24 for the AD7789.

REFERENCE INPUT

The AD7788/AD7789 has a fully differential input capability for

the channel. The common-mode range for these differential

inputs is from GND to VDD. The reference input is unbuffered

and, therefore, excessive R-C source impedances will introduce

gain errors. The reference voltage REFIN (REFIN(+) –

REFIN(–)) is 2.5 V nominal, but the AD7788/AD7789 is func-

tional with reference voltages from 0.1 V to VDD. In applications

where the excitation (voltage or current) for the transducer on

the analog input also drives the reference voltage for the part,

the effect of the low frequency noise in the excitation source

will be removed because the application is ratiometric. If the

AD7788/AD7789 is used in a nonratiometric application, a low

noise reference should be used.

Recommended 2.5 V reference voltage sources for the AD7788/

AD7789 include the ADR381 and ADR391, which are low noise,

low power references. If the analog circuitry uses a 2.5 V power

supply, the reference voltage source will require some head-

room. In this case, a 2.048 V reference such as the ADR380 or

ADR390 can be used. Again, these are low power, low noise

references. Also note that the reference inputs provide a high

impedance, dynamic load. Because the input impedance of each

reference input is dynamic, resistor/capacitor combinations on

these inputs can cause dc gain errors, depending on the output

impedance of the source that is driving the reference inputs.

Rev. 0 | Page 17 of 20

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