AD6659 datasheet(19/40 Pages) AD

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AD6659

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VOLTAGE REFERENCE

A stable and accurate 1.0 V voltage reference is built into the

AD6659. The VREF can be configured using either the internal

1.0 V reference or an externally applied 1.0 V reference voltage.

The various reference modes are summarized in the sections

that follow. The Reference Decoupling section describes best

practices for PCB layout of the reference.

Internal Reference Connection

A comparator within the AD6659 detects the potential at the

SENSE pin and configures the reference in one of two possible

modes, which are summarized in Table 10. If SENSE is grounded,

the reference amplifier switch is connected to the internal resistor

divider (see Figure 33), setting VREF to 1.0 V.

VREF

SENSE

0.5V

ADC

SELECT

LOGIC

0.1µF

1.0µF

VIN–A/VIN–B

VIN+A/VIN+B

ADC

CORE

Figure 33. Internal Reference Configuration

If the internal reference of the AD6659 is used to drive multiple

converters to improve gain matching, the loading of the reference

by the other converters must be considered. Figure 34 shows

how the internal reference voltage is affected by loading.

–3.0

LOAD CURRENT (mA)

–0.5

–1.0

–1.5

–2.0

–2.5

0.2

0.4

0.6

0.8

1.0

1.4

1.6

1.8

1.2

INTERNAL VREF = 0.993V

Figure 34. VREF Accuracy vs. Load Current

External Reference Operation

The use of an external reference may be necessary to enhance

the gain accuracy of the ADC or improve thermal drift charac-

teristics. Figure 35 shows the typical drift characteristics of the

internal reference in 1.0 V mode.

–1

–2

–3

–4

–5

–6

–40

–20

TEMPERATURE (°C)

VREF ERROR (mV)

Figure 35. Typical VREF Drift

When the SENSE pin is tied to AVDD, the internal reference is

disabled, allowing the use of an external reference. An internal

reference buffer loads the external reference with an equivalent

7.5 kΩ load (see Figure 25). The internal buffer generates the

positive and negative full-scale references for the ADC core.

Therefore, the external reference must be limited to a maximum

of 1.0 V.

Table 10. Reference Configuration Summary

Selected Mode

SENSE Voltage (V)

Resulting VREF (V)

Resulting Differential Span (V p-p)

Fixed Internal Reference

AGND to 0.2

1.0 internal

2.0

Fixed External Reference

AVDD

1.0 applied to external VREF pin

2.0

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