AD7294 Analog Devices, AD7294 Datasheet - Page 23

• Manufacturer Part Number: AD7294
• Description: 12-Bit Monitor and Control System with Multichannel ADC, DACs, Temperature Sensor, and Current Sense
• Manufacturer: Analog Devices

Specifications of AD7294

• Resolution (bits): 12bit
• # Chan: 9
• Sample Rate: 200kSPS
• Interface: I²C/Ser 2-Wire,Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (Vref) p-p,2 V p-p,Uni (Vref),Uni (Vref) x 2
• Adc Architecture: SAR
• Pkg Type: CSP

Data Sheet
gain. Therefore, for an input voltage of ±200 mV at the pins, an
output span of ±2.5 V is generated.
The current sensors on the
any flicker noise and offset present in the sensed signal. This is
achieved by implementing a chopping technique that is transpa-
rent to the user. The V
the analog inputs to the amplifiers are then swapped, and the
differential voltage is once again converted by the AD7294. The
two conversion results enable the digital removal of any offset
or noise. Switches on the amplifier inputs enable this chopping
technique to be implemented. This process requires 6 μs in total
to return a final result.
Choosing R
The resistor values used in conjunction with the current sense
amplifiers on the
cation requirements in terms of voltage, current, and power.
Small resistors minimize power dissipation, have low inductance
to prevent any induced voltage spikes, and have good tolerance,
which reduce current variations. The final values chosen are a
compromise between low power dissipation and good accuracy.
Low value resistors have less power dissipated in them, but higher
value resistors may be required to utilize the full input range of
the ADC, thus achieving maximum SNR performance.
When the sense current is known, the voltage range of the
AD7294
sense current to yield a suitable shunt value. If the power dissi-
pation in the shunt resistor is too large, the shunt resistor can
be reduced, in which case, less of the ADC input range is used.
Using less of the ADC input range results in conversion results,
which are more susceptible to noise and offset errors because
offset errors are fixed and are thus more significant when
smaller input ranges are used.
R
pation rating of the resistor is exceeded, its value may drift or
the resistor may be damaged resulting in an open circuit. This
can result in a differential voltage across the terminals of the
AD7294
protection is afforded to the current sensors on the
the recommended current limiting resistors, RF1 and RF2, as
illustrated in Figure 43. The
continuous current of 30 mA; thus, an RF2 of 1 kΩ provides
adequate protection for the AD7294.
If I
choose a resistor with low inductance. Low inductance metal
film resistors are best suited for these applications.
Current Sense Filtering
In some applications, it may be desirable to use external
filtering to reduce the input bandwidth of the amplifier (see
Figure 43). The −3 dB differential bandwidth of this filter is
equal to
SENSE
SENSE
BW
must be able to dissipate the I
has a large high frequency component, take care to
DM
current sensor (200 mV) is divided by the maximum
in excess of the absolute maximum ratings. Additional
= 1/(4πRC)
SENSE
AD7294
SENSE
signal is first converted by the AD7294,
are determined by the specific appli-
AD7294
AD7294
2
are designed to remove
R losses. If the power dissi-
can handle a maximum
AD7294
Rev. H | Page 23 of 48
by
Note that the maximum series resistance on the RS(+) and
RS(−) inputs (as shown in Figure 42) is limited to a maximum
of 1 kΩ due to back-to-back ESD protection diodes from RS(+)
and RS(−) to V
with R1 and R2 (shown in Figure 42), it affects the gain of the
amplifier. Any mismatch between RF1 and RF2 can introduce
offset error.
For certain RF applications, the optimum value for RF1 and
RF2 is 1 kΩ whereas CF1 can range from 1 μF to 10 μF. CF2 is a
decoupling capacitor for the V
dependant, but for initial evaluation, values in the range of 1 nF
to 100 nF are recommended.
Kelvin Sense Resistor Connection
When using a low value sense resistor for high current
measurement, the problem of parasitic series resistance can
arise. The lead resistance can be a substantial fraction of the
rated resistance, making the total resistance a function of lead
length. Avoid this problem by using a Kelvin sense connection.
This type of connection separates the current path through the
resistor and the voltage drop across the resistor. Figure 44 shows
the correct way to connect the sense resistor between the RS(+)
and RS(−) pins of the AD7294.
ANALOG COMPARATOR LOOP
The
for independent analog control. This circuitry enables users
to quickly detect if the sensed voltage across the shunt has
FLOW FROM
CURRENT
Figure 44. Kelvin Sense Connections (RSX Can Be Either RS1 or RS2)
AD7294
SUPPLY
Figure 43. Current Sense Filtering (RS
TRACES
KELVIN
SENSE
AD7294
10nF
contains two setpoint comparators that are used
PP
V
V
PP
PP
. Also, note that if RF1 and RF2 are in series
RSX(+)
SENSE RESISTOR
AD7294
PP
RSx(+)
supply. Its value is application
RF1
RSX(–)
X
Can Be Either RS1 or RS2)
R
SENSE
CF
RF2
RSx(–)
I
LOAD
CURRENT
FLOW TO
LOAD
AD7294