AD4696 datasheet(29/96 Pages) AD

29 / 96 page

Data Sheet

AD4695/AD4696

Rev. 0 | Page 29 of 96

INx+1

REFGND

COM

VREF/2

INx

Figure 67. Channel Configuration Option B

INx+1

REFGND

COM

VREF/2

INx

Figure 68. Channel Configuration Option C

Analog Input High-Z Mode

To achieve optimal data sheet performance from traditional

high resolution multiplexed SAR ADCs, system designers must

often include dedicated, high bandwidth, low noise ADC driver

amplifiers between the analog signal conditioning circuitry and

the ADC inputs to settle the voltage kickback that occurs at the

analog inputs between conversions. The AD4695/AD4696

analog input high-Z mode simplifies the design requirements

of the AFE circuitry that drives the analog inputs and facilitates

the design of small footprint, high channel density, precision

multiplexed SAR ADC signal chains.

Analog input high-Z mode significantly reduces the magnitude

of the voltage kickback that occurs at the analog inputs when

the ADC and multiplexer switches reconnect at the start of the

ADC acquisition phase (see the Signal Settling Requirements

section). Figure 20 shows the voltage kickback that occurs on

an analog input driven to 5 V after switching from another

analog input driven to 0 V with analog input high-Z mode

disabled and enabled.

The reduction in the voltage kickback increases the effective

input impedance of the AD4695/AD4696 analog inputs and

reduces the bandwidth requirements of the AFE circuitry to

achieve desired settling accuracy and performance. The relaxed

bandwidth requirements of the AD4695/AD4696 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

degradation caused by series resistance between the front-end

amplifiers and the AD4695/AD4696 analog inputs, which allows

the resistor in the external RC filter (shown as REXT in Figure 64

and Figure 107) 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 69 and Figure 70 demonstrate how a lower power, lower

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

performance as a lower noise, higher bandwidth ADC driver

amplifier (ADA4807-1) by utilizing the AD4695/AD4696

analog input high-Z mode. Figure 69 and Figure 70 show the

SNR and THD performance of the AD4695/AD4696 paired

with the ADA4077-1 and ADA4807-1 with various external RC

filter components with analog input high-Z mode disabled and

enabled. Figure 71 shows the circuit configuration used to measure

the performance metrics shown in Figure 69 and Figure 70. The

standard sequencer is configured to alternate between two

AD4695/AD4696 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 11 shows the bandwidth,

input noise, and supply current 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 71, analog input high-Z mode removes the

need for an ADA4807-1 or equivalent ADC driver amplifier for

each of its 16 analog input channels, which reduces the standby

current consumption of the system by roughly 16 mA, and

drastically reduces the full solution footprint.

Table 23 provides a list of recommended companion amplifiers

and external RC filter components to pair with the AD4695/

AD4696 for different target sample rates and input signal

bandwidths.

AD4696 Datasheet(PDF) 29 Page - Analog Devices

AD4696 Datasheet(HTML) 29 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

Datasheet Download

Link URL