AD9656EBZ datasheet(24/47 Pages) AD

24 / 47 page

Data Sheet

AD9656

Rev. A | Page 23 of 46

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 51), setting the voltage at the VREF pin, VREF, to

1.0 V. If SENSE is connected to an external resistor divider (see

Figure 52), VREF is defined as











 





REF

where:

7 kΩ ≤ (R1 + R2) ≤ 10 kΩ

VREF

SENSE

0.5V

AD9656

SELECT

LOGIC

0.1µF

1.0µF

VINx–

VINx+

ADC

CORE

Figure 51. 1.0 V Internal Reference Configuration

SENSE

VREF

0.5V

AD9656

SELECT

LOGIC

0.1µF

1.0µF

VINx–

VINx+

ADC

CORE

Figure 52. Programmable Internal Reference Configuration

If the internal reference of the AD9656 drives multiple converters

to improve gain matching, the loading of the reference by the other

converters must be considered. Figure 53 and Figure 54 show

how the internal reference voltage is affected by loading.

–5

–4

–3

–2

–1

0.5

1.0

1.5

2.0

2.5

LOAD CURRENT (mA)

INTERNAL VREF = 1.0V

Figure 53. VREF Error (Internal VREF = 1.0 V) vs. Load Current

–12

–10

–8

–6

–4

–2

0.5

1.0

1.5

2.0

2.5

LOAD CURRENT (mA)

INTERNAL VREF = 1.4V

Figure 54. VREF Error (Internal VREF = 1.4 V) vs. Load Current

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