hi7191 Intersil Corporation, hi7191 Datasheet - Page 11

• Manufacturer Part Number: hi7191
• Description: 24-bit, High Precision, Sigma Delta A/d Converter
• Manufacturer: Intersil Corporation

of the integrator goes into the comparator. The output of the
comparator is then fed back via a 1-bit DAC to the summing
junction. The feedback loop forces the average of the fed
back signal to be equal to the input signal V
Analog Inputs
The analog input on the HI7191 is a fully differential input
with programmable gain capabilities. The input accepts both
unipolar and bipolar input signals and gains range from 1 to
128. The common mode range of this input is from AV
AV
voltage lies within the power supplies. The input impedance
of the HI7191 is dependent upon the modulator input
sampling rate and the sampling rate varies with the selected
PGIA gain. Table 3 below shows the sampling rates and
input impedances for the different gain settings of the
HI7191. Note that this table is valid only for a 10MHz master
clock. If the input clock frequency is changed then the input
impedance will change accordingly. The equation used to
calculate the input impedance is:
Z
where C
the modulator sampling rate.
Bipolar/Unipolar Input Ranges
The input on the HI7191 can accept either unipolar or bipolar
input voltages. Bipolar or unipolar options are chosen by
programming the B/U bit of the Control Register.
Programming the part for either unipolar or bipolar operation
does not change the input signal conditioning.
The inputs are differential, and as a result are referenced to
the voltage on the V
+1.25V and the HI7191 is configured for unipolar operation
with a gain of 1 and a V
IN
8, 16, 32, 64, 128
DD
= 1/(C
V
IN
FIGURE 6. SIMPLE MODULATOR BLOCK DIAGRAM
TABLE 3. EFFECTIVE INPUT IMPEDANCE vs GAIN
provided that the absolute value of the analog input
GAIN
in
IN
1
2
4
is the nominal input capacitance (8pF) and f
x f
+
-
S
∑
),
INLO
PGIA
SAMPLING RATE
REF
input. For example, if V
78.125
156.25
DAC
(kHz)
312.5
of +2.5V, the input voltage range
625
11
V
INTEGRATOR
V
RLO
RHI
∫
INPUT IMPEDANCE
IN
COMPARATOR
.
(MΩ)
1.6
0.8
0.4
0.2
+
-
INLO
SS
is
S
is
to
HI7191
on the V
and the HI7191 is configured for bipolar mode with gain of 1
and a V
input is -1.25V to +3.75V.
Programmable Gain Instrumentation Amplifier
The Programmable Gain Instrumentation Amplifier allows the
user to directly interface low level sensors and bridges directly
to the HI7191. The PGIA has 4 selectable gain options of 1, 2,
4, 8 which are implemented by multiple sampling of the input
signal. Input signals can be gained up further to 16, 32, 64 or
128. These higher gains are implemented in the digital section
of the design to maintain a high signal to noise ratio through
the front end amplifiers. The gain is digitally programmable in
the Control Register via the serial interface. For optimum
PGIA performance the V
of the analog supplies.
Differential Reference Input
The reference inputs of the of the HI7191, V
provide a differential reference input capability. The nominal
differential voltage (V
common mode voltage cab be anywhere between AV
AV
degradation in performance with the maximum reference
voltage being V
be used but performance will be degraded since the LSB
size is reduced.
The full scale range of the HI7191 is defined as:
FSR
FSR
and V
operation of the device.
The reference inputs provide a high impedance dynamic
load similar to the analog inputs and the effective input
impedance for the reference inputs can be calculated in the
same manner as it is for the analog input impedance. The
only difference in the calculation is that C
inputs is 10.67pF. Therefor, the input impedance range for
the reference inputs is from 149kΩ in a gain of 8 or higher
mode to 833kΩ in the gain of 1 mode.
V
The voltage at the V
analog circuitry is referenced to and should always be tied to
the midpoint of the AV
provides a common mode input voltage for the internal
operational amplifiers and must be driven from a low noise,
low impedance source if it is not tied to analog ground.
Failure to do so will result in degraded HI7191 performance.
It is recommended that V
operating off of AV
V
of the power supplies which is limited by the common mode input
CM
CM
DD
BIPOLAR
UNIPOLAR
also determines the headroom at the upper and lower ends
. Larger values of V
RHI
Input
REF
INHI
must always be greater than V
= 2 x V
of +2.5V, the analog input range on the V
input is +1.25V to +3.75V. If V
= V
REF
REF
DD
REF
= +5V. Smaller values of V
CM
/GAIN
REF
DD
= +5V and AV
/GAIN
CM
input is the voltage that the internal
CM
REF
and AV
= V
pin should be tied to the mid point
be tied to analog ground when
RHI
can be used without
SS
- V
SS
supplies. This point
RLO
= -5V supplies.
) is +2.5V and the
RLO
IN
INLO
for the reference
RHI
for proper
REF
and V
is +1.25V
June 1, 2006
can also
SS
FN4138.8
INHI
RLO
and
,