11

in a gain-of-two configuration. An external, user-supplied,

0.1

analog ground is used to set the dominant pole and to

maintain the stability of the operational amplifier.

The HI5767 is designed to accept a +2.5V reference voltage

differential analog input voltage range of

±0.5V.

The user does have the option of supplying an external

+2.5V reference voltage. As a result of the high input

typically, the external reference voltage being used is only

required to source 1mA of reference input current. In the

situation where an external reference voltage will be used

an external 0.1

µF capacitor must be connected from the

the stability of the internal operational amplifier.

In order to minimize overall converter noise it is

recommended that adequate high frequency decoupling be

Analog Input, Differential Connection

The analog input to the HI5767 is a differential input that can

be configured in various ways depending on the signal

source and the required level of performance. A fully

differential connection (Figure 17 and Figure 18) will deliver

the best performance from the converter.

Since the HI5767 is powered by a single +5V analog supply,

the analog input is limited to be between ground and +5V.

For the differential input connection this implies the analog

input common mode voltage can range from 0.25V to 4.75V.

The performance of the ADC does not change significantly

with the value of the analog input common mode voltage.

available to the user to help simplify circuit design when using

an AC coupled differential input. This low output impedance

voltage source is not designed to be a reference but makes an

excellent DC bias source and stays well within the analog

input common mode voltage range over temperature.

For the AC coupled differential input (Figure 17) and with

Conversely, the converter will be at negative full scale when

The analog input can be DC coupled (Figure 18) as long as

the inputs are within the analog input common mode voltage

range (0.25V

≤ VDC ≤ 4.75V).

The resistors, R, in Figure 18 are not absolutely necessary

but may be used as load setting resistors. A capacitor, C,

frequency noise on the inputs, also improving performance.

Values around 20pF are sufficient and can be used on AC

coupled inputs as well. Note, however, that the value of

capacitor C chosen must take into account the highest

frequency component of the analog input signal.

Analog Input, Single-Ended Connection

The configuration shown in Figure 19 may be used with a

single ended AC coupled input.

voltage equal to VDC. The converter will be at positive full scale

= -0.5V). Sufficient headroom must be provided such that the

input voltage never goes above +5V or below AGND. In this case,

VDC could range between 0.5V and 4.5V without a significant

change in ADC performance. The simplest way to produce VDC

The single ended analog input can be DC coupled

(Figure 20) as long as the input is within the analog input

common mode voltage range.

HI5767

R

R

FIGURE 16. AC COUPLED DIFFERENTIAL INPUT

HI5767

R

R

C

VDC

VDC

FIGURE 17. DC COUPLED DIFFERENTIAL INPUT

HI5767

VDC

R

FIGURE 18. AC COUPLED SINGLE ENDED INPUT

HI5767