AD7787BRMZ Datasheet AD7787BRMZ, AD7787BRMZ-RL, AD7787BRM-REEL | P19-P21

AD7787 Data Sheet

Rev. A | Page 18 of 20

CIRCUIT DESCRIPTION

ANALOG INPUT CHANNEL

The AD7787 has two analog input channels that are connected

to the on-chip buffer amplifier when the device is operated in

buffered mode and directly to the modulator when the device is

operated in unbuffered mode. In buffered mode (the BUF bit in

the mode register is set to 1), the input channel feeds into a high

impedance input stage of the buffer amplifier. Therefore, the

input can tolerate significant source impedances and is tailored

for direct connection to external resistive-type sensors, such as

strain gauges or resistance temperature detectors (RTDs).

When BUF = 0, the part is operated in unbuffered mode.

This results in a higher analog input current. 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 15

shows the allowable external resistance/capacitance values for

the unbuffered mode such that no gain error at the 20-bit level

is introduced.

Table 15. External R-C Combination for No 20-Bit Gain

Error

C (pF) R (Ω)

50 16.7 k

100 9.6 k

500 2.2 k

1000 1.1 k

5000 160

The absolute input voltage range in buffered mode is restricted

to a range between GND + 100 mV and V

− 100 mV. Care

must be taken in setting up the common-mode voltage so that

these limits are not exceeded. Otherwise, there is degradation in

linearity and noise performance.

The absolute input voltage in unbuffered mode includes the

range between GND − 100 mV and V

+ 30 mV resulting

from being unbuffered. 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 AD7787 can accept either unipolar or

bipolar input voltage ranges. Unipolar and bipolar signals on

the AIN1(+) input are referenced to the voltage on the AIN(−)

input. For example, if AIN1(−) is 2.5 V and the ADC is

configured for unipolar mode, the input voltage range on the

AIN1(+) pin is 2.5 V to 5 V when REFIN = 2.5 V. If the ADC is

configured for bipolar mode, the analog input range on the

AIN1(+) input is 0 V to 5 V.

The voltage on AIN2 is referenced to GND. Therefore, when

bipolar mode is selected and the part is operated in unbuffered

mode, the voltage on AIN2 can vary from GND − 100 mV to

+2.5 V. In unipolar mode, the voltage on AIN2 can vary from

0 V to 2.5 V. The bipolar/unipolar option is chosen by

programming the 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

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





REF

VAINCode /2



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









1/2





REF

VAINCode

where AIN is the analog input voltage and N = 24.

REFERENCE INPUT

The AD7787 has a single-ended reference that is 2.5 V nominal,

but the AD7787 is functional with reference voltages from 0.1 V

to V

. 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 is removed because the application is

ratiometric. If the AD7787 is used in a nonratiometric

application, a low noise reference should be used.

Recommended 2.5 V reference voltage sources for the AD7787

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

power references. In a system that operates from a 2.5 V power

supply, the reference voltage source requires some headroom. In

this case, a 2.048 V reference, such as the ADR380, can be used,

requiring only 300 mV of headroom. Also note that the

reference input provides a high impedance, dynamic load.

Because the input impedance of the reference input is dynamic,

resistor/capacitor combinations on this input can cause dc gain

errors, depending on the output impedance of the source

driving the reference inputs.

Data Sheet AD7787

Rev. A | Page 19 of 20

Reference voltage sources like those previously recommended,

e.g., ADR391, will typically have low output impedances and

are, therefore, tolerant to having decoupling capacitors on

REFIN without introducing gain errors in the system. Deriving

the reference input voltage across an external resistor means

that the reference input sees a significant external source

impedance. External decoupling on the REFIN pin is not

recommended in this type of circuit configuration.

MONITOR

Along with converting external voltages, the AD7787 can

monitor the voltage on the V

pin. When the CH1 and CH0

bits in the communications register are set to 1, the voltage on

the V

pin is internally attenuated by 5 and the resultant

voltage is applied to the Σ-Δ modulator using an internal

1.17 V reference for the analog-to-digital conversion. This is

useful because variations in the power supply voltage can be

monitored.

GROUNDING AND LAYOUT

The digital filter provides rejection of broadband noise on the

power supply, except at integer multiples of the modulator

sampling frequency. The digital filter also removes noise from

the analog and reference inputs, provided that these noise

sources do not saturate the analog modulator. As a result, the

AD7787 is more immune to noise interference than a

conventional high resolution converter. However, because the

resolution of the AD7787 is so high, and the noise levels from

the AD7787 are so low, care must be taken with regard to

grounding and layout.

The printed circuit board that houses the AD7787 should be

designed such that the analog and digital sections are separated

and confined to certain areas of the board. A minimum etch

technique is generally best for ground planes because it gives

the best shielding.

It is recommended that the AD7787’s GND pin be tied to the

AGND plane of the system. In any layout, it is important that

the user keep in mind the flow of currents in the system,

ensuring that the return paths for all currents are as close as

possible to the paths the currents took to reach their

destinations. Avoid forcing digital currents to flow through the

AGND sections of the layout.

The AD7787’s ground plane should be allowed to run under the

AD7787 to prevent noise coupling. The power supply lines to

the AD7787 should use as wide a trace as possible to provide

low impedance paths and reduce the effects of glitches on the

power supply line. Fast switching signals, such as clocks, should

be shielded with digital ground to avoid radiating noise to other

sections of the board, and clock signals should never be run

near the analog inputs. Avoid crossover of digital and analog

signals. Traces on opposite sides of the board should run at

right angles to each other. This reduces the effects of

feedthrough through the board. A microstrip technique is by far

the best, but it is not always possible with a double-sided board.

In this technique, the component side of the board is dedicated

to ground planes, while signals are placed on the solder side.

Good decoupling is important when using high resolution

ADCs. V

should be decoupled with 10 μF tantalum in parallel

with 0.1 μF capacitors to GND. To achieve the best from these

decoupling components, they should be placed as close as

possible to the device, ideally right up against the device. All

logic chips should be decoupled with 0.1 μF ceramic capacitors

to DGND.

APPLICATIONS

Battery Monitoring

In battery monitoring, the battery current and voltage are

measured. The current is passed through a 100 μΩ resistor.

Because the current is from −200 A to +2000 A, the result is a

voltage from −20 mV to +200 mV. Channel AIN1 of the

AD7787 can be connected directly to the shunt resistor to

measure this current. The battery voltage can vary from 12 V to

42 V with peaks up to 60 V. This voltage is attenuated using an

external resistor network before being applied to the AD7787.

The buffers onboard the AD7787 mean that channel AIN2 can

be connected directly to the high impedance attenuator circuit

without introducing gain errors.

04477-0-016

-

ADC

AIN1(–)

AIN1(+)

SHUNT

100

–200A TO

+2000A

GND REFIN

AD7787

DOUT/RDY

DIN

SCLK

SERIAL

INTERFACE

AND

LOGIC

CONTROL

MUX

AIN2

12V OR 42V

(60V PEAK)

–

TTENUATION

CIRCUIT

Figure 16. Battery Monitoring

AD7787 Data Sheet

Rev. A | Page 20 of 20

OUTLINE DIMENSIONS

COMPLIANT TO JEDEC STANDARDS MO-187-BA

091709-A

6°

0°

0.70

0.55

0.40

0.50 BSC

0.30

0.15

1.10 MAX

3.10

3.00

2.90

COPLANARITY

0.10

0.23

0.13

3.10

3.00

2.90

5.15

4.90

4.65

PIN 1

IDENTIFIER

15° MAX

0.95

0.85

0.75

0.15

0.05

Figure 17. 10-Lead Mini Small Outline Package [MSOP]

(RM-10)

Dimensions shown in millimeters

ORDERING GUIDE

Models

Temperature Range Package Description Package Option Branding

AD7787BRM −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C1T

AD7787BRMZ −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C42

AD7787BRM-REEL −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C1T

AD7787BRMZ-RL −40°C to +105°C 10-Lead Mini Small Outline Package (MSOP) RM-10 C42

EVAL-AD7787EB Evaluation Board

Z = RoHS Compliant Part.

registered trademarks are the property of their respective owners.

D04477-0-3/13(A)

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21

AD7787BRMZ

Mfr. #:

Buy AD7787BRMZ

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC Low Pwr 2-Ch 24-Bit

Lifecycle:

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AD7787BRMZ

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