ADAS3022BCPZ datasheet(27/40 Pages) AD

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Data Sheet

ADAS3022

Rev. A | Page 27 of 40

IN0+

COM–

IN0

IN1+

IN2+

IN3+

IN4+

IN5+

IN6+

IN7+

IN0+

IN1+

IN2+

IN3+

IN4+

IN5+

IN6+

IN7+

IN1

IN2

IN3

IN4

IN5

IN6

IN7

COM

IN0

IN1

IN2

IN3

IN4

IN5

IN6

IN7

COM

A—8 CHANNELS,

SINGLE-ENDED

B—8 CHANNELS,

COMMON REFERENCE

IN2+

IN3+

IN4+

IN5+

IN0

IN1

IN2

IN3

IN4

IN5

IN6

IN7

COM

IN0

IN1

IN2

IN3

IN4

IN5

IN6

IN7

COM

IN0+ (–)

C—4 CHANNELS,

DIFFERENTIAL

D—COMBINATION

COM–

IN0– (+)

IN1+ (–)

IN1– (+)

IN0+ (–)

IN0– (+)

IN1+ (–)

IN1– (+)

IN2+ (–)

IN2– (+)

IN3+ (–)

IN3– (+)

Figure 61. Multiplexed Analog Input Configurations

Channel Sequencer

The ADAS3022 includes a channel sequencer that is useful for

scanning channels in a repeated fashion. Refer to the Channel

Sequencer Details section for more information.

Auxiliary Input Channel

The ADAS3022 includes an auxiliary input channel pair (AUX+

and AUX−) that bypasses the mux and PGIA stages, allowing direct

access to the SAR ADC core for applications where the additional

dedicated channel pair is required. As detailed previously, the

inputs are protected only from AVDD and AGND because the high

voltage supplies are used for the mux and PGIA stages but not the

lower voltage ADC core.

When the source impedance of the driving circuit is low, the

AUX± inputs can be driven directly. Large source impedances

significantly affect the ac performance, especially THD. The dc

performance parameters are less sensitive to the input impedance.

The maximum source impedance depends on the amount of

THD that can be tolerated. The THD degrades as a function of

the source impedance and the maximum input frequency.

Driver Amplifier Choice

For systems that cannot drive AUX± directly, a suitable op amp

buffer should be used to preserve the ADAS3022 performance.

The driver amplifier must meet the following requirements:

•

The noise generated by the driver amplifier must be kept as

low as possible to preserve the SNR and the transition noise

performance of the ADAS3022. The noise from the

amplifier is filtered by the ADAS3022 analog input circuit

or by an external filter, if one is used. Because the typical

noise of the ADAS3022’s SAR ADC core is 35 µV rms (VREF =

4.096 V), the SNR degradation due to the amplifier is













−

)

(

log

3dB

LOSS

SNR

where:

f−3dB is the input bandwidth (8 MHz) of the ADAS3022’s SAR

ADC core expressed in megahertz or the cutoff frequency of

an input filter, if one is used.

N is the noise gain of the amplifier (for example, 1 in buffer

configuration).

eN is the equivalent input noise voltage of the op amp

expressed in nV/√Hz.

•

For ac applications, the driver should have a THD performance

commensurate with the ADAS3022.

•

The analog input circuit must settle a full-scale step onto

the capacitor array at a 16-bit level (0.0015%). In amplifier

data sheets, settling at 0.1% to 0.01% is more commonly

specified. This may differ significantly from the settling

time at a 16-bit level and should be verified prior to driver

selection.

Table 8. Recommended Driver Amplifiers

Amplifier

Typical Application

ADA4841-1, ADA4841-2

Very low noise, small, and low power

ADA4897-1, ADA4897-2

Very low noise, low and high frequencies

AD8655

5 V single supply, low noise

AD8021, AD8022

Very low noise and high frequency

OP184

Low power, low noise, and low frequency

AD8605, AD8615

5 V single supply, low power

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