REV. B
–20–
AD7853/AD7853L
POWER VS. THROUGHPUT RATE
The main advantage of a full power-down after a conversion is
that it significantly reduces the power consumption of the part
at lower throughput rates. When using this mode of operation,
the AD7853 is only powered up for the duration of the conver-
sion. If the power-up time of the AD7853 is taken to be 5 µs
and it is assumed that the current during power-up is 4 mA typ,
then power consumption as a function of throughput can easily
be calculated. The AD7853 has a conversion time of 4.6 µs
with a 4 MHz external clock. This means the AD7853 con-
sumes 4 mA typ, (or 12 mW typ V
DD
= 3 V) for 9.6 µs in every
conversion cycle if the device is powered down at the end of a
conversion. If the throughput rate is 1 kSPS, the cycle time is
1000 µs and the average power dissipated during each cycle is
(9.6/1000) × (12 mW) = 115 µW. The graph, Figure 24, shows
the power consumption of the AD7853 as a function of through-
put. Table VII lists the power consumption for various through-
put rates.
Table VII. Power Consumption vs. Throughput
Throughput Rate Power
1 kSPS 115 µW
10 kSPS 1.15 mW
POWER – mW
0.01
1
10
THROUGHPUT – kSPS
0
10 20 30 40 50
0.1
515253545
AD7853 (4MHz CLK)
AD7853L (1.8MHz CLK)
Figure 26. Power vs. Throughput Rate
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 AD7853/
AD7853L have a number of calibration features that may be
required in some applications and there are a number of advan-
tages in performing these different types of calibration. First, the
internal errors in the ADC can be reduced significantly to give
superior dc performance; and second, system offset and gain
errors can be removed. 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 AD7853/AD7853L by ad-
justing the analog input range of the part for a specific system.
There are two main calibration modes on the AD7853/AD7853L,
self-calibration and system calibration. There are various op-
tions in both self-calibration and system calibration as outlined
previously in Table III. 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 1. 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 AD7853 with a 4 MHz master clock.
These calibration times are master clock dependent. Therefore
the calibration times for the AD7853L (CLKIN = 1.8 MHz)
will be larger than those quoted in Table VIII.
Table VIII. Calibration Times (AD7853 with 4 MHz CLKIN)
Type of Self- or System Calibration Time
Full 31.25 ms
Gain + Offset 6.94 ms
Offset 3.47 ms
Gain 3.47 ms
Automatic Calibration on Power-On
The CAL pin has a 0.15 µA 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
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 32 ms
(4 MHz CLKIN). Therefore the autocalibration should be
complete before operating the part. After calibration, the part is
accurate to the 12-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 a four different calibration options within the self-
calibration mode. There is a full self-calibration where the DAC,
internal offset, and internal gain errors are calibrated out. Then,
there is the (Gain + Offset) self-calibration which calibrates 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
