AD4697BCPZ datasheet(70/107 Pages) AD

70 / 107 page

Data Sheet

AD4697/AD4698

APPLICATIONS INFORMATION

analog.com

Rev. 0 | 70 of 107

ANALOG FRONT-END DESIGN

The AFE companion circuitry for the AD4697/AD4698 normally

includes an external RC filter and an ADC driver or a precision

operational amplifier between the signal being measured and the

AD4697/AD4698 analog inputs.

The component selection and design of the AFE circuitry driving the

AD4697/AD4698 analog inputs have a direct impact on the overall

system performance. The AFE must be designed with the system

target noise, accuracy, distortion, and settling requirements of the

end application. The following sections provide recommendations

for designing AFE and signal conditioning circuits based on these

requirements.

External RC Filter

The external RC low-pass filter consists of an external resistor and

capacitor (represented by REXT and CEXT in Figure 66 and

Figure 112). These components act to reduce the wideband noise

from the AFE circuitry, reduce the nonlinear voltage kickback that

occurs at the analog inputs, and protect the analog inputs from

overvoltage events. Selecting the appropriate REXT and CEXT val-

ues for these functions is described in the Analog Front-End Noise

Considerations section, the Signal Settling Requirements section,

and the Analog Input Overvoltage Protection section.

Ensure that the CEXT capacitor is an NP0 ceramic capacitor to limit

distortion artifacts, and that the PCB layout minimizes the parasitic

impedance between CEXT and the analog input pin. See the Layout

Guidelines section for more information.

Signal Settling Requirements

As described in the Converter Operation and Analog Inputs sec-

tions, the AD4697/AD4698 analog inputs (IN0 to IN7 and COM) are

routed to the ADC core inputs via the internal analog multiplexer.

As shown in Figure 66, the ADC core capacitive DAC can be

represented by a switched capacitive load.

At the start of the conversion phase, the multiplexer switches are

disconnected and the voltage on the currently selected analog input

channel is sampled on the capacitive DAC. During the acquisition

phase, the multiplexer switches (SWMUX+ and SWMUX−) close to

connect the next selected analog input channel to the capacitive

DAC. A voltage glitch (commonly referred to as kickback) occurs

when these switches close due to the difference between the

voltage on the capacitive DAC and the voltage on the selected

analog input pins.

To achieve the specified performance of the AD4697/AD4698, this

kickback must be settled to within half an LSB of the ADC core

before the start of the next conversion phase (that is, the next

CNV rising edge). The rate at which the kickback voltage is settled

depends on the transient characteristics and bandwidth of the AFE

circuitry. Signal settling requirements therefore dictate the minimum

allowable AFE bandwidth and constrain the driver amplifier and

external RC filter selection.

Table 26 provides a list of recommended amplifiers and external RC

filter components for various sample rates and signal bandwidths.

Figure 71 and Figure 72 in the Analog Input High-Z Mode section

show SNR and THD performance with various amplifiers and exter-

nal RC component values.

Analog input high-Z mode significantly reduces the bandwidth re-

quirements of the AFE by minimizing the size of the voltage kick-

back. Figure 21 shows the difference in magnitude of the kickback

when analog input high-Z mode is disabled and enabled.

Analog Front-End Noise Considerations

The magnitude of the AFE noise directly impacts the dynamic range

and SNR performance of the overall AD4697/AD4698 signal chain.

Select the AFE components and configuration to achieve the target

noise specification for the overall system.

Figure 113 illustrates the primary noise sources in a typical analog

front-end driver circuit.

Figure 113. Noise Sources in Typical ADC AFE Circuit

AD4697BCPZ Datasheet(PDF) 70 Page - Analog Devices

AD4697BCPZ Datasheet(HTML) 70 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL