ADC08D1000EVAL datasheet(30/37 Pages) NSC

30 / 37 page

2.0 Applications Information

(Continued)

reduced to 75% of the values indicated. In the Enhanced

Control Mode, these values will be determined by the full

scale range and offset settings in the Control Registers.

TABLE 5. DIFFERENTIAL INPUT TO OUTPUT

RELATIONSHIP (Non-Extended Control Mode, FSR

High)

IN+VIN−

Output Code

CM − 217.5mV

CM + 217.5mV

0000 0000

CM − 109 mV

CM + 109 mV

0100 0000

0111 1111 /

1000 0000

CM + 109 mV

CM −109 mV

1100 0000

CM + 217.5mV

CM − 217.5mV

1111 1111

The buffered analog inputs simplify the task of driving these

inputs and the RC pole that is generally used at sampling

ADC inputs is not required. If it is desired to use an amplifier

circuit before the ADC, use care in choosing an amplifier with

adequate noise and distortion performance and adequate

gain at the frequencies used for the application.

Note that a precise d.c. common mode voltage must be

present at the ADC inputs. This common mode voltage,

CMO, is provided on-chip when a.c. input coupling is used

and the input signal is a.c. coupled to the ADC.

When the inputs are a.c. coupled, the V

CMO output must be

grounded, as shown in Figure 11. This causes the on-chip

CMO voltage to be connected to the inputs through on-chip

50k-Ohm resistors.

IMPORTANT NOTE: An Analog input channel that is not

used (e.g. in DES Mode) should be left floating when the

inputs are a.c. coupled. Do not connect an unused analog

input to ground.

When the d.c. coupled mode is used, a common mode

voltage must be provided at the differential inputs. This

common mode voltage should track the V

CMO output pin.

Note that the V

CMO output potential will change with tem-

perature. The common mode output of the driving device

should track this change.

IMPORTANT NOTE: An analog input channel that is not

used (e.g. in DES Mode) should be tied to the V

CMO voltage

when the inputs are d.c coupled. Do not connect unused

analog inputs to ground.

Full-scale distortion performance falls off rapidly as the

input common mode voltage deviates from V

CMO. This is

a direct result of using a very low supply voltage to

minimize power. Keep the input common voltage within

50 mV of V

CMO.

Performance is as good in the d.c. coupled mode as it is

in the a.c. coupled mode, provided the input common

mode voltage at both analog inputs remain within 50 mV

of V

CMO.

If d.c. coupling is used, it is best to servo the input common

mode voltage, using the V

CMO pin, to maintain optimum

performance. An example of this type of circuit is shown in

Figure 12.

One such circuit should be used in front of the V

IN+ input and

another in front of the V

IN− input. In that figure, RD1,RD2 and

D3 are used to divide the VCMO potential so that, after being

gained up by the amplifier, the input common mode voltage

is equal to V

CMO from the ADC. RD1 and RD2 are split to

allow the bypass capacitor to isolate the input signal from

CMO.RIN,RD2 and RD3 will divide the input signal, if nec-

essary. Capacitor "C" in Figure 12 should be chosen to keep

any component of the input signal from affecting V

CMO.

Be sure that the current drawn from the V

CMO output does

not exceed 100 µA.

The Input impedance in the d.c. coupled mode (V

CMO pin not

grounded) consists of a precision 100

Ω resistor between

IN+ and VIN− and a capacitance from each of these inputs

to ground. In the a.c. coupled mode the input appears the

same except there is also a resistor of 50K between each

analog input pin and the V

CMO potential.

Driving the inputs beyond full scale will result in a saturation

or clipping of the reconstructed output.

20097444

FIGURE 11. Differential Input Drive

20097455

FIGURE 12. Example of Servoing the Analog Input with

CMO

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