AD4697 datasheet(74/107 Pages) AD

74 / 107 page

Data Sheet

AD4697/AD4698

APPLICATIONS INFORMATION

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Rev. 0 | 74 of 107

Reference Circuit Design for Driving REF Input

Figure 116 shows a typical connection diagram for the reference

circuitry driving the REF input of the AD4697/AD4698.

The device driving the REF input must have sufficiently low output

impedance so that the reference input current does not cause VREF

to deviate enough to violate the system performance targets. To

achieve data sheet performance, VREF must remain within half an

LSB. The maximum output impedance of the device driving the

REF input (RO_MAX) is therefore:

RO_MAX=VREF216+1

IREF

(15)

where IREF is the average REF input current.

Most voltage references specify load regulation in ppm/mA, which

can be converted to effective output impedance with the following:

LMAX=1000×RO_MAXVREF

(16)

whereLMAX is the load regulation specification for the voltage refer-

ence in ppm/mA that corresponds to the calculated RO_MAX.

IREF is typically 11 µA at 1 MSPS with reference input high-Z mode

enabled and 320 µA at 1 MSPS with reference input high-Z mode

disabled (in unipolar mode). IREF scales linearly with the ADC sam-

ple rate (see Table 1 and Figure 40). The output impedance and

load regulation requirements of the reference circuitry are therefore

less strict at lower sample rates. Table 27 shows the calculated

RO_MAX and LMAX for VREF = 5 V and for different sample rates and

with reference input high-Z mode disabled and enabled. Table 27

also provides recommendations for voltage references and discrete

reference buffers for each of these conditions.

Note when driving the REF input on the WLCSP option of the

AD4698, the internal reference buffer must be disabled, and the ref-

erence buffer bypass switch must be open (see SWBP in Figure 74).

The internal reference buffer is disabled by default (REFBUF_EN =

0), and the SWBP is open by default (REFBUF_BP = 0).

Figure 116. Typical Connection Diagram for Driving REF Input

Table 27. Reference Circuitry Recommendations, REF Input

Sample Rate

Reference Input

High-Z Mode

IREF (µA)

RO_MAX (Ω)

LMAX (ppm/mA)

Recommended Voltage References and Reference Buffers

1 MSPS

Disabled

320

0.12

ADR4550 with ADA4807-1, ADR445 with ADA4807-1, ADR435

1 MSPS

Enabled

3.2

640

ADR4550, ADR445, ADR435

500 kSPS

Disabled

160

0.24

ADR445 with ADA4807-1, ADR4550, ADR435

500 kSPS

Enabled

6.4

320

ADR4550, ADR445, ADR435

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