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Data Sheet

AD9656

VOLTAGE REFERENCE

A stable and accurate voltage reference is built into the AD9656.

VREF can be configured using the internal 1.0 V reference, using

an externally applied 1.0 V to 1.4 V reference voltage, or using

an external resistor divider applied to the internal reference to

produce a user-selectable reference voltage. The reference modes

are described in the Internal Reference Connection section and the

External Reference Operation section. Externally bypass the

VREF pin to ground with a low equivalent series resistance

(ESR), 1.0 µF capacitor in parallel with a low ESR, 0.1 µF

ceramic capacitor.

Internal Reference Connection

A comparator within the AD9656 detects the potential at the

SENSE pin and configures the reference for one of three

possible modes, which are summarized in Table 11. If SENSE is

grounded, the reference amplifier switch is connected to the

internal resistor divider (see Figure 50), setting the voltage at the

VREF pin, VREF, to 1.0 V. If SENSE is connected to an external

resistor divider (see Figure 51), VREF is defined as











 +

VREF

where:

7 kΩ ≤ (R1 + R2) ≤ 10 kΩ

Figure 50. 1.0 V Internal Reference Configuration

Figure 51. Programmable Internal Reference Configuration

If the internal reference of the AD9656 is used to drive multiple

converters to improve gain matching, the loading of the reference

by the other converters must be considered. Figure 52 and

Figure 53 show how the internal reference voltage is affected

by loading.

Figure 52. VREF Error (Internal VREF = 1.0 V) vs. Load Current

Figure 53. VREF Error (Internal VREF = 1.4 V) vs. Load Current

VREF

SENSE

0.5V

AD9656

SELECT

LOGIC

0.1µF

1.0µF

VINx–

VINx+

ADC

CORE

SENSE

VREF

0.5V

AD9656

SELECT

LOGIC

0.1µF

1.0µF

VINx–

VINx+

ADC

CORE

–5

–4

–3

–2

–1

0.5

1.0

1.5

2.0

2.5

LOAD CURRENT (mA)

INTERNAL VREF = 1.0V

–12

–10

–8

–6

–4

–2

0.5

1.0

1.5

2.0

2.5

LOAD CURRENT (mA)

INTERNAL VREF = 1.4V

Rev. 0 | Page 23 of 44

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