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3-12
An internal band-gap reference voltage followed by an
amplier/buffer generates the precision +2.5V reference
voltage used by the converter. A band-gap reference circuit
is used to generate a precision +1.25V internal reference
voltage. This voltage is then amplied by a wide-band
uncompensated operational amplier connected in a
gain-of-two conguration. An external, user-supplied, 0.1
F
capacitor connected from the VROUT output pin to analog
ground is used to set the dominant pole and to maintain the
stability of the operational amplier.
Reference Voltage Input, VREFIN
The HI5662 is designed to accept a +2.5V reference voltage
source at the VRIN input pin. Typical operation of the
converter requires VRIN to be set at +2.5V. The HI5662 is
tested with VRIN connected to VROUT yielding a fully
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
impedance presented at the VRIN input pin, 1.25k typically,
the external reference voltage being used is only required to
source 2mA of reference input current. In the situation where
an external reference voltage will be used an external 0.1
F
capacitor must be connected from the VROUT output pin to
analog ground in order to maintain the stability of the internal
operational amplier.
In order to minimize overall converter noise it is
recommended that adequate high frequency decoupling be
provided at the reference voltage input pin, VRIN.
Analog Input, Differential Connection
The analog input of the HI5662 is a differential input that can
be congured in various ways depending on the signal
source and the required level of performance. A fully
differential connection (Figure 15 and Figure 16) will deliver
the best performance from the converter.
Since the HI5662 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 signicantly
with the value of the analog input common mode voltage.
A DC voltage source, I/QVDC, equal to 3.0V (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 15) and with
VRIN connected to VROUT, 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 I/QIN+ input is at VDC + 0.25V and
the I/QIN- input is at VDC - 0.25V (I/QIN+ - I/QIN- = +0.5V).
Conversely, the converter will be at negative full scale when
the I/QIN+ input is equal to VDC - 0.25V and I/QIN- is at
VDC + 0.25V (I/QIN+ - I/QIN- = -0.5V).
The analog input can be DC coupled (Figure 16) as long as
the inputs are within the analog input common mode voltage
range (0.25V
≤ VDC ≤ 4.75V).
The resistors, R, in Figure 16 are not absolutely necessary
but may be used as load setting resistors. A capacitor, C,
connected from I/QIN+ to I/QIN- will help lter any high
frequency noise on the inputs, also improving performance.
Values around 20pF are sufcient 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 conguration shown in Figure 17 may be used with a
single ended AC coupled input.
Again, with VRIN connected to VROUT, if VIN is a 1VP-P
sinewave, then I/QIN+ is a 1.0VP-P sinewave riding on a
positive voltage equal to VDC. The converter will be at positive
full scale when I/QIN+ is at VDC + 0.5V (I/QIN+ - I/QIN- = +0.5V)
and will be at negative full scale when I/QIN+ is equal to
VDC - 0.5V (I/QIN+ - I/QIN- = -0.5V). Sufficient headroom must
be provided such that the input voltage never goes above +5V
I/QIN+
I/QVDC
I/QIN-
HI5662
VIN
-VIN
R
FIGURE 15. AC COUPLED DIFFERENTIAL INPUT
I/QIN+
I/QVDC
I/QIN-
HI5662
VIN
-VIN
R
C
VDC
FIGURE 16. DC COUPLED DIFFERENTIAL INPUT
I/QIN+
I/QIN-
HI5662
VIN
VDC
R
FIGURE 17. AC COUPLED SINGLE ENDED INPUT
HI5662