AD7738 datasheet(25/28 Pages) AD

25 / 28 page

REV. 0

AD7738

–25–

Figure 16. Typical ADC Frequency Response

NORMALIZED INPUT FREQUENCY

(INPUT FREQUENCY

CONVERSION TIME)

0.1

–10

–20

–30

–40

–50

–60

CHOP = 1

a. Chopping Enabled

NORMALIZED INPUT FREQUENCY

(INPUT FREQUENCY

CONVERSION TIME)

0.1

1000

–10

–20

–30

–40

–50

–60

100

CHOP = 0

b. Chopping Disabled

Analog Inputs Voltage Range

The absolute input voltage range with input the buffer enabled

is restricted from AGND + 200 mV to AVDD – 300 mV, which

also places restrictions on the common-mode range. Care must

be taken in setting up the common-mode voltage and input

voltage range so that these limits are not exceeded, otherwise

there will be degradation in linearity performance.

The analog inputs on the AD7738 can accept either unipolar or

bipolar input voltage ranges. Bipolar input ranges do not imply

that the part can handle negative voltages with respect to system

ground on its analog inputs. Unipolar and bipolar signals on the

AIN(+) input are referenced to the voltage on the respective

AIN(–) input.

For example, if AINCOM is 2.5 V and CH0 is configured to

measure AIN0 – AINCOM, 0 V to 1.25 V, the input voltage

range on the AIN0 input is 2.5 V to 3.75 V. If CH0 is config-

ured to measure AIN0 – AINCOM,

±1.25 V, the input voltage

range on the AIN0 input is 1.25 V to 3.75 V.

Analog Inputs Extended Voltage Range

The AD7738 output data code span corresponds to the nominal

input voltage range. However, the correct operation of the ADC

is guaranteed within the min/max input voltage range.

When the CLAMP bit of the Mode register is set to 1, the

Channel Data register will be digitally clamped either to all

zeros or all ones when the analog input voltage goes outside

the nominal input voltage range.

As shown in Tables XIII and XIV, when CLAMP = 0, the data

reflect the analog input voltage outside the nominal voltage

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

Status register should be considered along with the Data register

value to decode the actual conversion result.

Table XIII. Input Voltage Range

1.25 V, 16 Bits, CLAMP = 0

Input (V)

Data (Hex)

SIGN

OVR

+1.45000

147B

+1.25008

0001

+1.25004

0000

+1.25000

FFFF

+0.00004

8001

0.00000

8000

–0.00004

7FFF

–1.25000

0000

–1.25004

FFFF

–1.25008

FFFE

–1.45000

EB85

Table XIV. Input Voltage Range 0 V to 1.25 V, 16 Bits,

CLAMP = 0

Input (V)

Data (Hex)

SIGN

OVR

1.45000

28F5

1.25004

0001

1.25002

0000

1.25000

FFFF

0.00002

0001

0.00000

0000

–0.00002

0000

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