AD677 データシートの表示（PDF） - Analog Devices

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AD677

16-Bit 100 kSPS Sampling ADC

Analog Devices 

AD677

made for selecting one with low noise. A capacitor connected

between REF IN and AGND will reduce the demands on the

reference by decreasing the magnitude of high frequency com-

ponents required to be sourced by the reference.

Figures 6 and 7 represent typical design approaches.

+12V

CN

1.0µF

2

VIN

8

6

AD586

4

10µF

VREF

0.1µF AD677

AGND

Figure 6.

Figure 6 shows a voltage reference circuit featuring the 5 V out-

put AD586. The AD586 is a low cost reference which utilizes a

buried Zener architecture to provide low noise and drift. Over

the 0°C to +70°C range, the AD586M grade exhibits less than

1.0 mV output change from its initial value at +25°C. A noise

reduction capacitor, CN, reduces the broadband noise of the

AD586 output, thereby optimizing the overall performance of

the AD677. It is recommended that a 10 µF to 47 µF high qual-

ity tantalum capacitor and a 0.1 µF capacitor be tied between

the VREF input of the AD677 and ground to minimize the im-

pedance on the reference.

Using the AD677 with ± 10 V input range (VREF = 10 V) typi-

cally requires ± 15 V supplies to drive op amps and the voltage

reference. If ± 12 V is not available in the system, regulators

such as 78L12 and 79L12 can be used to provide power for the

AD677. This is also the recommended approach (for any input

range) when the ADC system is subjected to harsh environ-

ments such as where the power supplies are noisy and where

voltage spikes are present. Figure 7 shows an example of such a

system based upon the 10 V AD587 reference, which provides a

300 µV LSB. Circuitry for additional protection against power

supply disturbances has been shown. A 100 µF capacitor at each

10Ω

10µF

0.1µF

AD587

2 VIN

VO 6

GND NR 8

4

1µF

+15V

100µF

78L12

+5V

100µF

–15V

100µF

79L12

10Ω

0.01µF

10Ω

10Ω

0.01µF

10µF

0.1µF

0.1µF

VCC

VDD

VREF

AD677

VEE

VIN

10µF

0.1µF

10µF 0.1µF

VIN

Figure 7.

regulator prevents very large voltage spikes from entering the

regulators. Any power line noise which the regulators cannot

eliminate will be further filtered by an RC filter (10 Ω/10 µF)

having a –3 dB point at 1.6 kHz. For best results the regulators

should be within a few centimeters of the AD677.

ANALOG INPUT

As previously discussed, the analog input voltage range for the

AD677 is ± VREF. For purposes of ground drop and common

mode rejection, the VIN and VREF inputs each have their own

ground. VREF is referred to the local analog system ground

(AGND), and VIN is referred to the analog ground sense pin

(AGND SENSE) which allows a remote ground sense for the

input signal.

The AD677 analog inputs (VIN, VREF and AGND SENSE) ex-

hibit dynamic characteristics. When a conversion cycle begins,

each analog input is connected to an internal, discharged 50 pF

capacitor which then charges to the voltage present at the corre-

sponding pin. The capacitor is disconnected when SAMPLE is

taken LOW, and the stored charge is used in the subsequent

conversion. In order to limit the demands placed on the external

source by this high initial charging current, an internal buffer

amplifier is employed between the input and this capacitance for

a few hundred nanoseconds. During this time the input pin ex-

hibits typically 20 kΩ input resistance, 10 pF input capacitance

and ± 40 µA bias current. Next, the input is switched directly to

the now precharged capacitor and allowed to fully settle. During

this time the input sees only a 50 pF capacitor. Once the sample

is taken, the input is internally floated so that the external input

source sees a very high input resistance and a parasitic input

capacitance of typically only 2 pF. As a result, the only domi-

nant input characteristic which must be considered is the high

current steps which occur when the internal buffers are switched

in and out.

In most cases, these characteristics require the use of an external

op amp to drive the input of the AD677. Care should be taken

with op amp selection; even with modest loading conditions,

most available op amps do not meet the low distortion require-

ments necessary to match the performance capabilities of the

AD677. Figure 8 represents a circuit, based upon the AD845,

which will provide excellent overall performance.

For applications optimized more for low distortion and low

noise, the AD845 of Figure 8 may be replaced by the AD743.

1k Ω

±5V

INPUT

1k Ω

499 Ω

+12V

2

7

AD845

3

4

0.1µF

6

0.1µF

–12V

AD677

VIN

AGND

AGND

SENSE

Figure 8.

REV. A

–11–

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