AD7856 Analog Devices, AD7856 Datasheet - Page 21

• Manufacturer Part Number: AD7856
• Description: 5 V Single-Supply, 8-Channel, 14-Bit, 285 kSPS, Serial Sampling ADC
• Manufacturer: Analog Devices

Specifications of AD7856

• Resolution (bits): 14bit
• # Chan: 8
• Sample Rate: 285kSPS
• Interface: Ser,SPI
• Analog Input Type: SE-Uni
• Ain Range: (Vref) p-p,Uni (Vref)
• Adc Architecture: SAR
• Pkg Type: DIP,SOIC,SOP

CALIBRATION SECTION
Calibration Overview
The automatic calibration that is performed on power up en-
sures that the calibration options covered in this section will not
be required in a significant amount of applications. The user
will not have to initiate a calibration unless the operating condi-
tions change (CLKIN frequency, analog input mode, reference
voltage, temperature, and supply voltages). The AD7856 has a
number of calibration features that may be required in some
applications and there are a number of advantages in performing
these different types of calibration. First, the internal errors in
the ADC can be reduced significantly to give superior dc perfor-
mance, and secondly, system offset and gain errors can be re-
moved. This allows the user to remove reference errors (whether
it be internal or external reference) and to make use of the full
dynamic range of the AD7856 by adjusting the analog input
range of the part for a specific system.
There are two main calibration modes on the AD7856, self-
calibration and system calibration. There are various options in
both self-calibration and system calibration as outlined previ-
ously in Table IV. All the calibration functions can be initiated
by pulsing the CAL pin or by writing to the control register and
setting the STCAL bit to one. The timing diagrams that follow
involve using the CAL pin.
The duration of each of the different types of calibrations is
given in Table VIII for the AD7856 with a 6 MHz master clock.
These calibration times are master clock dependent.
Automatic Calibration on Power-On
The CAL pin has a 0.15 A pull up current source connected to
it internally to allow for an automatic full self-calibration on
power-on. A full self-calibration will be initiated on power-on if
a capacitor is connected from the CAL pin to DGND. The
REV. A
Table VIII. Calibration Times (AD7856 with 6 MHz CLKIN)
Figure 24. Power vs. Throughput Rate (6 MHz CLK)
100
0.1
10
1
0
Type of Self- or
System Calibration
Full
Offset + Gain
Offset
Gain
10
THROUGHPUT – kSPS
20
30
Time
41.7 ms
9.26 ms
4.63 ms
4.63 ms
40
50
–21–
internal current source connected to the CAL pin charges up
the external capacitor and the time required to charge the exter-
nal capacitor will depend on the size of the capacitor itself. This
time should be large enough to ensure that the internal refer-
ence is settled before the calibration is performed. A 33 nF
capacitor is sufficient to ensure that the internal reference has
settled (see Power-Up Times) before a calibration is initiated
taking into account trigger level and current source variations on
the CAL pin. However, if an external reference is being used,
this reference must have stabilized before the automatic calibra-
tion is initiated (a larger capacitor on the CAL pin should be
used if the external reference has not settled when the autocali-
bration is initiated). Once the capacitor on the CAL pin has
charged, the calibration will be performed which will take 42 ms
(6 MHz CLKIN). Therefore the autocalibration should be
complete before operating the part. After calibration, the part is
accurate to the 14-bit level and the specifications quoted on the
data sheet apply. There will be no need to perform another
calibration unless the operating conditions change or unless a
system calibration is required.
Self-Calibration Description
There are four different calibration options within the self-
calibration mode. First, there is a full self-calibration where the
DAC, internal gain, and internal offset errors are calibrated out.
Then, there is the (Gain + Offset) self-calibration which cali-
brates out the internal gain error and then the internal offset
errors. The internal DAC is not calibrated here. Finally, there
are the self-offset and self-gain calibrations which calibrate out
the internal offset errors and the internal gain errors respectively.
The internal capacitor DAC is calibrated by trimming each of
the capacitors in the DAC. It is the ratio of these capacitors to
each other that is critical, and so the calibration algorithm en-
sures that this ratio is at a specific value by the end of the cali-
bration routine. For the offset and gain there are two separate
capacitors, one of which is trimmed when an offset or gain cali-
bration is performed. Again, it is the ratio of these capacitors to
the capacitors in the DAC that is critical and the calibration
algorithm ensures that this ratio is at a specified value for both
the offset and gain calibrations.
The zero-scale error is adjusted for an offset calibration, and the
positive full-scale error is adjusted for a gain calibration.
Self-Calibration Timing
The diagram of Figure 25 shows the timing for a full self-
calibration. Here the BUSY line stays high for the full length of
the self-calibration. A self-calibration is initiated by bringing the
CAL pin low (which initiates an internal reset) and then high
again or by writing to the control register and setting the STCAL
bit to 1 (note that if the part is in a power-down mode the CAL pulse -
width must take account of the power-up time ). The BUSY line is
triggered high from the rising edge of CAL (or the end of the
write to the control register if calibration is initiated in soft-
ware), and BUSY will go low when the full-self calibration is
complete after a time t
For the self- (gain + offset), self-offset and self-gain calibrations
the BUSY line will be triggered high by the rising edge of the
CAL signal (or the end of the write to the control register if
calibration is initiated in software) and will stay high for the
full duration of the self calibration. The length of time that
the BUSY is high will depend on the type of self-calibration that
CAL
as shown in Figure 25.
AD7856