AD4134 datasheet(59/92 Pages) AD

59 / 92 page

Data Sheet

AD4134

APPLICATIONS INFORMATION

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Rev. 0 | 59 of 92

Figure 96. Internal LDO Regulator Mode Power Connections

The internal LDO regulators are enabled only when the IOVDD

supply is powered up first by an external 1.8 V supply.

The internal LDO regulators work properly if the power supply

sequence in Figure 97 is followed. Ensure that the IOVDD and

LDOIN pins are powered after DVDD5, as shown in Figure 97.

Figure 97. Power Sequencing in Internal LDO Mode

If the internal LDO regulators are not used, tie the LDOIN pin to

DVDD1V8, as shown in Figure 98.

Figure 98. External Power Mode Connections

If AVDD1V8, DVDD1V8, and CLKVDD are powered from a sepa-

rate external supply, take caution on the supply sequencing. All

three supplies are connected internally through the back diode of

the regulator. If one supply powers up first, it can supply power

to other supplies through the back diode and the other LDO regula-

tors.

REFERENCE NOISE FILTERING

The user can reduce the noise contribution of the reference source

to the overall ADC conversion accuracy by filtering the reference

signal. An internal 20 Ω resistor between the REFIN pin and the

REFCAP pin enables the user to form a first-order RC filter by

connecting a capacitor on the REFCAP pin.

Figure 99. Reference Input Connection Using REFIN Pin

The equivalent noise bandwidth of a first-order filter is 0.25/RC in

Hz.

The noise contribution of the reference source is proportional to the

ADC input signal. The reference noise contribution is at the highest

when the input signal is at full scale. The reference noise has no

impact on the output when the ADC inputs are shorted.

As a general rule, limit the reference noise to ¼ of the noise of the

ADC to have a minimal effect on the overall SNR.

The total reference noise is the root sum square of its 1/f noise and

its wideband noise.

The 1/f noise of the reference can be estimated by its peak-to-peak

noise specification over the 0.1 Hz to 10 Hz frequency range. The

wideband noise can be calculated from the voltage noise density

specification of the reference and the reference noise bandwidth.

An example to calculate the reference noise requirement based on

the ADC mode of operation follows.

Consider the AD4134 device that is operating in high performance

mode, ODR = 374 kSPS, and wideband 0.433 × ODR filter with a

reference voltage of 4.096 V.

According to Table 9, the ADC noise in this setup is 12.63 µV rms.

The reference noise is ¼, equal to 3.16 µV rms.

An ADR444 reference IC is chosen to provide the reference voltage

for the AD4134. The ADR444 has a 0.1 Hz to 10 Hz peak noise

of 1.8 µV p-p, and a noise spectrum density of 78.6 nV/√Hz. The

ADR444 1/f noise is 1.8 µV p-p or 1.8/6.6 = 0.273 µV rms.

The total reference noise is the root sum square of its 1/f noise and

its wideband noise. Therefore,

√(0.2732 + n2WB) < 3.16

Solving the equation yields the wideband noise, nWB, of the

ADR444, which must be less than 3.14 µV rms.

The wideband noise of the ADR444 can be calculated by multiply-

ing its noise spectrum density by the square root of the noise

bandwidth.

AD4134 Datasheet(PDF) 59 Page - Analog Devices

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