AD7739 datasheet(29/32 Pages) AD

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AD7739

Rev. 0 | Page 29 of 32

ANALOG INPUT’S EXTENDED VOLTAGE RANGE

The AD7739 output data code span corresponds to the nominal

input voltage range. The ADC is functional outside the nominal

input voltage range, but the performance might degrade. The

sigma-delta modulator was designed to fully cover 16% analog

input overrange; outside this range, the performance might

degrade more rapidly.

When the CLAMP bit in the mode register is set to 1, the

channel data register will be digitally clamped to either all 0s or

all 1s when the analog input voltage goes outside the nominal

input voltage range.

As shown in Table 14 and Table 15, when CLAMP = 0, the data

reflects the analog input voltage outside the nominal voltage

range. In this case, the SIGN and OVR bits in the channel status

to decode the actual conversion result.

Note that the OVR bit in the channel status register is generated

digitally from the conversion result and indicates the sigma-

delta modulator (nominal) overrange. The OVR bit DOES NOT

indicate exceeding the AIN pin’s absolute voltage limits.

Table 14. Extended Input Voltage Range,

Nominal Voltage Range ±1.25 V, 16 Bits, CLAMP = 0

Input (V)

Data (hex)

SIGN

OVR

+1.45000

0x147B

+1.25008

0x0001

+1.25004

0x0000

+1.25000

0xFFFF

+0.00004

0x8001

0.00000

0x8000

–0.00004

0x7FFF

–1.25000

0x0000

–1.25004

0xFFFF

–1.25008

0xFFFE

–1.45000

0xEB85

Table 15. Extended Input Voltage Range,

Nominal Voltage Range +1.25 V, 16 Bits, CLAMP = 0

Input (V)

Data (hex)

SIGN

OVR

+1.45000

0x28F5

+1.25004

0x0001

+1.25002

0x0000

+1.25000

0xFFFF

+0.00002

0x0001

+0.00000

0x0000

–0.00002

0x0000

VOLTAGE REFERENCE INPUTS

The AD7739 has a differential reference input, REF IN(+) and

REF IN(–). The common-mode range for these inputs is from

AGND to AVDD. The nominal differential reference voltage for

specified operation is 2.5 V. Both reference inputs feature

dynamic load. Therefore, the reference inputs should be

connected to a low impedance reference voltage source.

External resistance/capacitance combinations may result in gain

errors on the part.

The output noise performance outlined in Table 4 through

Table 9 is for an analog input of 0 V and is unaffected by noise

on the reference. Obtaining the same noise performance as

shown in the noise tables over the full input range requires a

low noise reference source for the AD7739. If the reference

noise in the bandwidth of interest is excessive, it will degrade

the performance of the AD7739.

Recommended reference voltage sources for the AD7739

include the ADR421, AD780, REF43, and REF192.

REFERENCE DETECT

The AD7739 includes on-chip circuitry to detect if the part has

a valid reference for conversions. If the voltage between the

REFIN(+) and REFIN(–) pins goes below the NOREF trigger

voltage (0.5 V typ.) and the AD7739 is performing a conversion,

the NOREF bit in the channel status register is set.

I/O PORT

The AD7739 P0 pin can be used as a general-purpose digital

output or as a common analog input.

The P1 pin (SYNC/P1) can be used as a general-purpose digital

I/O pin or to synchronize the AD7739 with other devices in the

system. When the SYNC bit in the I/O port register is set and

the SYNC pin is low, the AD7739 does not process any

conversion. If it is put into single conversion mode, continuous

conversion mode, or any calibration mode, the AD7739 waits

until the SYNC pin goes high and then starts operation. This

allows conversion to start from a known point in time, i.e., the

rising edge of the SYNC pin. When configured as input, the

SYNC pin should be tied high or low.

The digital P0 and P1 voltage is referenced to the analog

supplies.

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