AD7858LBR datasheet(20/32 Pages) AD

20 / 32 page

REV. B

–20–

AD7858/AD7858L

The recommended value of the external capacitor is 100 nF;

this gives a power-up time of approximately 135 ms before a

calibration is initiated and normal operation should commence.

When CREF is fully charged, the power-up time from a hardware

or software power-down reduces to 5

µs. This is because an

internal switch opens to provide a high impedance discharge

path for the reference capacitor during power-down—see Figure

23. An added advantage of the low charge leakage from the

reference capacitor during power-down is that even though the

reference is being powered down between conversions, the

reference capacitor holds the reference voltage to within

0.5 LSBs with throughput rates of 100 samples/second and

over with a full power-down between conversions. A high input

impedance op amp like the AD707 should be used to buffer this

reference capacitor if it is being used externally. Note, if the

AD7858 is left in its power-down state for more than 100 ms,

the charge on CREF will start to leak away and the power-up

time will increase. If this long power-up time is a problem, the

user can use a partial power-down for the last conversion so the

reference remains powered up.

AD7858

REFIN/REFOUT

EXTERNAL

CAPACITOR

SWITCH OPENS

DURING POWER-DOWN

BUF

ON-CHIP

REFERENCE

TO OTHER

CIRCUITRY

Figure 23. On-Chip Reference During Power-Down

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 AD7858 is only powered up for the duration of the conver-

sion. If the power-up time of the AD7858 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 AD7858 has a conversion time of 4.6

µs

with a 4 MHz external clock. This means the AD7858 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 AD7858 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

AD7858 (4MHz CLK)

AD7858L (1.8MHz CLK)

Figure 24. 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 AD7858/

AD7858L 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 secondly, 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 AD7858/

AD7858L by adjusting the analog input range of the part for a

specific system.

There are two main calibration modes on the AD7858/AD7858L,

self-calibration and system calibration. There are various op-

tions in both self-calibration and system calibration as outlined

previously in Table IV. 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 AD7858 with a 4 MHz master clock.

These calibration times are master clock dependent. Therefore,

the calibrating times for the AD7858L (CLKIN = 1.8 MHz)

will be longer than those quoted in Table VIII.

Table VIII. Calibration Times (AD7858 with 4 MHz CLKIN)

Type of Self- or

System Calibration

Time

Full

31.25 ms

Offset + Gain

6.94 ms

Offset

3.47 ms

Gain

3.47 ms

AD7858LBR Datasheet(PDF) 20 Page - Analog Devices

AD7858LBR Datasheet(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Download

Link URL