AD7294 Analog Devices, AD7294 Datasheet - Page 25

• 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
Series Resistance Cancellation
The
effect of parasitic, base, and collector resistance on the tempera-
ture reading. This gives a more accurate result, without the need
for any user characterization of the parasitic resistance. The
AD7294
transparent to the user.
DAC OPERATION
The
control with 12 bits of resolution with a 2.5 V internal reference.
The DAC core is a thin film 12-bit string DAC with a 5 V output
span and an output buffer that can drive the high voltage output
stage. The DAC has a span of 0 V to 5 V with a 2.5 V reference
input. The output range of the DAC, which is controlled by the
offset input, can be positioned from 0 V to 15 V.
Resistor String
The resistor string structure is shown in Figure 48. It consists of
a string of 2
DAC register determines at which node on the string the voltage
is tapped off to be fed into the output amplifier. The voltage is
tapped off by closing one of the switches connecting the string
to the amplifier. This architecture is inherently monotonic,
voltage out, and low glitch. It is also linear because all of the
resistors are of equal value.
Output Amplifier
Referring to Figure 48, the purpose of A1 is to buffer the DAC
output range from 0 V to V
configured such that when an offset is applied to OFFSET IN x,
its output voltage is three times the offset voltage minus twice
the DAC voltage.
AD7294
AD7294
V
OUT
can compensate for up to 100 Ω in a process that is
= 3V
n
resistors, each of Value R. The code loaded to the
has been designed to automatically cancel out the
contains four 12-bit DACs that provide digital
R
R
R
R
R
OFFSET
Figure 48. Resistor String Structure
− 2V
DAC
REF
. The second amplifier, A2, is
TO OUTPUT
AMPLIFIER
Rev. H | Page 25 of 48
The DAC word is digitally inverted on-chip such that
where:
V
D is the decimal equivalent of the binary code that is loaded to the
DAC register.
n is the bit resolution of the DAC.
An example of the offset function is given in Table 8.
Table 8. Offset Voltage Function Example
Offset
Voltage
1.67 V
3.33 V
5.00 V
The user has the option of leaving the offset pin open, in which
case the voltage on the noninverting input of Op Amp A2 is set
by the resistor divider, giving
This generates the 5 V output span from a 2.5 V reference.
Digitally inverting the DAC allows the circuit to operate as a
generic DAC when no offset is applied. If the offset pin is not
being driven, it is best practice to place a 100 nF capacitor
between the pin and ground to improve both the settling time
and the noise performance of the DAC.
Note that a significant amount of power can be dissipated in the
DAC outputs. A thermal shutdown circuit sets the DAC outputs
to high impedance if a die temperature of >150°C is measured
by the internal temperature sensor. This also sets the overtem-
perature alert bit in Alert Register C, see the Alerts and Limits
Theory section. Note that this feature is disabled when the
temperature sensor powers down.
ADC AND DAC REFERENCE
The
2.5 V references, one for the ADCs and the other for the four
on-chip DACs. If the application requires an external reference,
it can be applied to the REF
REF
before being used by external circuitry. Decouple both the REF
REF
220 nF capacitor. On power-up, the AD7294 is configured for
use with an external reference. To enable the internal references,
write a zero to both the D4 and D5 bits in the power-down
register (see the Register Setting section for more details). Both
the ADC and DAC references require a minimum of 60 μs to
power up and settle to a 12-bit performance when a 220 nF
decoupling capacitor is used.
DAC
OUT
IN
AD7294
V
and V
V
is the output of the DAC before digital inversion.
DAC pin and the REF
OUT
OUT
/REF
= 3V
= 2V
DAC
IN
has two independent internal high performance
ADC pin. The internal reference should be buffered
=
OFFSET
DAC
V
0 V
5 V
10 V



V
OUT
REF
+ 2(V
with 0x000
×



DAC
2
D
OUT
OUT
n



− V
/REF



/REF
REF
IN
)
IN
ADC pin to AGND using a
DAC pin and/or to the
V
5 V − 1 LSB
10 V − 1 LSB
15 V − 1 LSB
OUT
with 0xFFF
AD7294
OUT
/