AD7853_15 datasheet(20/34 Pages) AD

20 / 34 page

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 VDD = 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

0.01

THROUGHPUT – kSPS

0.1

515

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

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

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