AD4697 datasheet(32/107 Pages) AD

32 / 107 page

Data Sheet

AD4697/AD4698

THEORY OF OPERATION

analog.com

Rev. 0 | 32 of 107

ments of the AD4697/AD4698 simplify the AFE circuit design by

broadening the selection of compatible amplifiers and external

RC filter components. Therefore, analog input high-Z mode helps

remove the requirement of dedicated ADC driver amplifiers per

channel, which significantly reduces system footprint and power

consumption.

The analog input high-Z mode also reduces performance degrada-

tion caused by series resistance between the front-end amplifiers

and the AD4697/AD4698 analog inputs, which allows the resistor in

the external RC filter (shown as REXT in Figure 66 and Figure 112)

to be larger compared to traditional multiplexed SAR ADCs. Using

larger REXT with smaller CEXT alleviates amplifier stability concerns

without significantly impacting distortion performance.

Figure 71 and Figure 72 demonstrate how a lower power, low-

er bandwidth amplifier (ADA4077-1) can achieve the same ac

performance as a lower noise, higher bandwidth ADC driver am-

plifier (ADA4807-1) by utilizing the AD4697/AD4698 analog input

high-Z mode. Figure 71 and Figure 72 show the SNR and THD

performance of the AD4697/AD4698 paired with the ADA4077-1

and ADA4807-1 with various external RC filter components with

analog input high-Z mode disabled and enabled. Figure 73 shows

the circuit configuration used to measure the performance metrics

shown in Figure 71 and Figure 72. The standard sequencer is con-

figured to alternate between two AD4697/AD4698 channels once

per conversion. The channels are driven by antiphase, full-scale,

1 kHz sine waves.

The ADA4807-1 is a low noise, high bandwidth amplifier that is

typically recommended for driving precision SAR ADCs, and the

ADA4077-1 is a high precision, low drift amplifier with a comparably

lower bandwidth. Table 13 shows the –3 dB bandwidth, input

voltage noise, and supply current per amplifier specifications for the

ADA4807-1 and ADA4077-1. When analog input high-Z mode is

disabled, the ADA4077-1 THD performance is degraded because

of its inability to settle the voltage kickback between conversions.

When analog input high-Z mode is enabled, the ADA4077-1 is able

to achieve similar THD performance to the ADA4807-1, despite

having a comparably lower bandwidth. In the example shown in

Figure 73, analog input high-Z mode removes the need for an

ADA4807-1 or equivalent ADC driver amplifier for each of its eight

analog input channels, which reduces the standby current con-

sumption of the system by roughly 8 mA, and drastically reduces

the full solution footprint.

Table 26 provides a list of recommended companion amplifiers and

external RC filter components to pair with the AD4697/AD4698 for

different target sample rates and input signal bandwidths.

Analog input high-Z mode is enabled with the AINHIZ_EN bit in the

CONFIG_INn registers. When the standard sequencer is enabled,

analog input high-Z mode is enabled or disabled for all eight

analog inputs and is set by the AINHIZ_EN bit in the CONFIG_IN0

two-cycle command mode or single-cycle command mode, analog

input high-Z mode is enabled or disabled for all eight analog inputs

independently and is set by the AINHIZ_EN bit in the corresponding

CONFIG_INn register for each input. Analog input high-Z mode is

always enabled when sampling the temperature sensor.

Analog input high-Z mode must be enabled when reference input

high-Z mode is enabled. If any analog input channels are config-

ured with analog input high-Z mode disabled, the reference input

high-Z mode must also be disabled.

Table 13. Companion Amplifier Specifications

Amplifier

Input Voltage

Noise

−3 dB

Bandwidth

Supply Current per

Amplifier

ADA4807-1/

3.1 nV/√Hz

180 MHz

1.0 mA

ADA4807-2/

ADA4807-4

ADA4077-1/

6.9 nV/√Hz

5.9 MHz

400 µA

ADA4077-2/

ADA4077-4

Figure 71. SNR vs. External RC Filter Components, Bandwidth for Various

Amplifiers (VREF = 5 V, fIN = 1 kHz)

Figure 72. THD vs. External RC Filter Components, Bandwidth for Various

Amplifiers (VREF = 5 V, fIN = 1 kHz)

AD4697 датащи(PDF) 32 Page - Analog Devices

AD4697 датащи(HTML) 32 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

датащи скачать

ссылки URL