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HI5667 Datasheet PDF : 10 Pages
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HI5667
Analog Input, Differential Connection
The analog input to the HI5667 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 4 and Figure 5) will deliver the
best performance from the converter.
Since the HI5667 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.
VIN
VIN+
R
HI5667
VDC
R
-VIN
VIN-
FIGURE 4. AC COUPLED DIFFERENTIAL INPUT
A DC voltage source, VDC, equal to 3.2V (typical), is made
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 4) and with
VREFIN connected to VREFOUT , full scale is achieved when
the VIN and -VIN input signals are 0.5VP-P, with -VIN being
180 degrees out of phase with VIN. The converter will be at
positive full scale when the VIN+ input is at VDC + 0.25V and
the VIN- input is at VDC - 0.25V (VIN+ - VIN- = +0.5V).
Conversely, the converter will be at negative full scale when
the VIN+ input is equal to VDC - 0.25V and VIN- is at
VDC + 0.25V (VIN+ - VIN- = -0.5V).
The analog input can be DC coupled (Figure 5) as long as
the inputs are within the analog input common mode voltage
range (0.25V ≤ VDC ≤ 4.75V).
VIN
VDC
VDC
-VIN
VIN+
R
C
HI5667
VDC
R
VIN-
FIGURE 5. DC COUPLED DIFFERENTIAL INPUT
The resistors, R, in Figure 5 are not absolutely necessary
but may be used as load setting resistors. A capacitor, C,
connected from VIN+ to VIN- will help filter any high
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 6 may be used with a
single ended AC coupled input.
VIN
VDC
VIN+
R
HI5667
VIN-
FIGURE 6. AC COUPLED SINGLE ENDED INPUT
Again, with VREFIN connected to VREFOUT, if VIN is a 1VP-P
sinewave, then VIN+ is a 1.0VP-P sinewave riding on a positive
voltage equal to VDC. The converter will be at positive full scale
when VIN+ is at VDC + 0.5V (VIN+ - VIN- = +0.5V) and will be at
negative full scale when VIN+ is equal to VDC - 0.5V (VIN+ - VIN-
= -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 is to use the DC bias source, VDC, output of the
HI5667.
The single ended analog input can be DC coupled (Figure 7)
as long as the input is within the analog input common mode
voltage range.
VIN
VDC
VIN+
R
C
HI5667
VDC
VIN-
FIGURE 7. DC COUPLED SINGLE ENDED INPUT
The resistor, R, in Figure 7 is not absolutely necessary but
may be used as a load setting resistor. A capacitor, C,
connected from VIN+ to VIN- will help filter any high
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.
A single ended source may give better overall system
performance if it is first converted to differential before
driving the HI5667.
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