Datasheet

AD7294 Data Sheet
Rev. H | Page 22 of 48
20kΩ
220kΩ
2 × V
REF
p-p
27Ω
27Ω
V+
V–
V+
V–
GND
2.5V
3.75V
1.25V
2.5V
3.75V
1.25V
REF
OUT
ADC
V
IN+
AD7294
1
V
IN–
440Ω
220Ω
0.47µF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
220Ω
220Ω
10kΩ
A
05747-023
Figure 39. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal
into a Differential Unipolar Signal
Pseudo Differential Mode
The four uncommitted analog input channels can be configured
as two pseudo differential pairs. Uncommitted input, V
IN
0 and
V
IN
1, are a pseudo differential pair, as are V
IN
2 and V
IN
3. In this
mode, V
IN+
is connected to the signal source, which can have a
maximum amplitude of V
REF
(or 2 × V
REF
, depending on the
range chosen) to make use of the full dynamic range of the part.
A dc input is applied to V
IN−
. The voltage applied to this input
provides an offset from ground or a pseudo ground for the V
IN+
input. Which channel is V
IN+
is determined by the ADC channel
allocation. The differential mode must be selected to operate in the
pseudo differential mode. The resulting converted pseudo differen-
tial data is stored in twos complement format in the result register.
The governing equation for the pseudo differential mode, for
V
IN
0 is
V
OUT
= 2(V
IN+
− V
IN−
) − V
REF_ADC
where V
IN+
is the single-ended signal and V
IN−
is a dc voltage.
The benefit of pseudo differential inputs is that they separate
the analog input signal ground from the ADC ground, allowing
dc common-mode voltages to be cancelled. The typical voltage
range for V
IN−
while in pseudo differential mode is shown in
Figure 40; Figure 41 shows a connection diagram for pseudo
differential mode.
2.0
1.5
1.0
0.5
–0.5
0
0 654321
V
IN–
(V)
V
REF
(V)
05747-095
AV
DD
= DV
DD
= 5V
V
DRIVE
= 5V
Figure 40. V
IN−
Input Range vs. V
REF
in Pseudo Differential Mode
DC INPUT
VOLTAGE
V
REF
p-p
REF
OUT
/REF
IN
ADC
V
IN+
AD7294
1
V
IN–
0.47µF
1
ADDITIONAL PINS OMITTED FOR CLARITY.
05747-026
Figure 41. Pseudo Differential Mode Connection Diagram
CURRENT SENSOR
Two bidirectional high-side current sense amplifiers are
provided that can accurately amplify differential current shunt
voltages in the presence of high common-mode voltages from
AV
DD
up to 59.4 V. Each amplifier can accept a ±200 mV
differential input. Both current sense amplifiers have a fixed
gain of 12.5 and utilize an internal 2.5 V reference.
An analog comparator is also provided with each amplifier for
fault detection. The threshold is defined as
1.2 × Full-Scale Voltage Range
When this limit is reached, the output is latched onto a
dedicated pin. This output remains high until the latch is
cleared by writing to the appropriate register.
AD7294
AV
DD
TO 54.5V
RS(+)
R
SENSE
I
LOAD
RS(–)
R1
40kΩ
R2
40kΩ
R3
100kΩ
R4
100kΩ
Q1 Q2
V
OUT
TO MUX
A1
A1
A2
V
PP
05747-029
Figure 42. High-Side Current Sense
The AD7294 current sense comprises two main blocks: a
differential and an instrumentation amplifier. A load current
flowing through the external shunt resistor produces a voltage
at the input terminals of the AD7294. Resistors R1 and R2
connect the input terminals to the differential amplifier (A1).
A1 nulls the voltage appearing across its own input terminals
by adjusting the current through R1 and R2 with Transistor Q1
and Transistor Q2. Common-mode feedback maintains the sum
of these currents at approximately 50 μA. When the input signal
to the AD7294 is zero, the currents in R1 and R2 are equal. When
the differential signal is nonzero, the current increases through
one of the resistors and decreases in the other. The current differ-
ence is proportional to the size and polarity of the input signal.
The differential currents through Q1 and Q2 are converted into
a differential voltage by R3 and R4. A2 is configured as an instru-
mentation amplifier, buffering this voltage and providing additional