AD5522JSVDZ datasheet(36/60 Pages) AD

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AD5522

Rev. A | Page 36 of 60

Table 14. References Suggested For Use with AD55221

Part No.

Voltage (V)

Initial

Accuracy %

Ref Out

TC (ppm/°C)

Ref Output

Current (mA)

Supply Voltage

Range (V)

Package

ADR435

0.04

+7 to +18

MSOP, SOIC

ADR445

0.04

+5.5 to +18

MSOP, SOIC

ADR431

2.5

0.04

+4.5 to +18

MSOP, SOIC

ADR441

2.5

0.04

+3 to +18

MSOP, SOIC

1 Subset of the possible references suitable for use with the AD5522. Visit www.analog.com for more options.

For other voltage and current ranges, the required reference

level can be calculated as follows:

Identify the nominal range required.

Identify the maximum offset span and the maximum gain

required on the full output signal range.

Calculate the new maximum output range, including the

expected maximum gain and offset errors.

Choose the new required VOUTMAX and VOUTMIN, keeping

the VOUT limits centered on the nominal values. Note that

AVDD and AVSS must provide sufficient headroom.

Calculate the value of VREF as follows:

VREF = (VOUTMAX − VOUTMIN)/4.5

Reference Selection Example

Nominal output range = 10 V (−2 V to +8 V)

Offset error = ±100 mV

Gain error = ±0.5%, and

REFGND = AGND = 0 V

Then

Gain error = ±0.5%

=> Maximum positive gain error = +0.5%

=> Output range including gain error = 10 V + 0.005(10 V) =

10.05 V

Offset error = ±100 mV

=> Maximum offset error span = 2(100 mV) = 0.2 V

=> Output range including gain error and offset error =

10.05 V + 0.2 V = 10.25 V

VREF calculation

Actual output range = 10.25 V, that is, −2.125 V to +8.125 V

(centered);

VREF = (8.125 V + 2.125 V)/4.5 = 2.28 V

If the solution yields an inconvenient reference level, the user

can adopt one of the following approaches:

•

Use a resistor divider to divide down a convenient, higher

reference level to the required level.

•

Select a convenient reference level above VREF and modify

the gain and offset registers to digitally downsize the reference.

In this way, the user can use almost any convenient refer-

ence level.

•

Use a combination of these two approaches.

In this case, the optimum reference is a 2.5 V reference; the user

can use the M and C registers and the offset DAC to achieve the

required −2 V to +8 V range. Change the ISENSE amplifier gain to

5 to ensure a full-scale current range of the specified values (see

the Current Range Selection section). This gain also allows opti-

mization of power supplies and minimizes power consumption

within the device.

It is important to bear in mind when choosing a reference value

that values other than 5 V (MI gain= 10) and 2.5 V (MI gain=

5) result in current ranges other than those specified. See the

Measure Current Gains section for more details.

CALIBRATION

Calibration involves determining the gain and offset of each

channel in each mode and overwriting the default values in the

M and C registers of the individual DACs. In some cases (for

example, FI mode), the calibration constants, particularly those

for gains, may be range dependent.

Reducing Zero-Scale Error

Zero-scale error can be reduced as follows:

Set the output to the lowest possible value.

Measure the actual output voltage and compare it to the

required value. This gives the zero-scale error.

Calculate the number of LSBs equivalent to the zero-scale

error and add this number to the default value of the C

reduced.

Reducing Gain Error

Gain error can be reduced as follows:

Measure the zero-scale error.

Set the output to the highest possible value.

Measure the actual output voltage and compare it to the

required value. This is the gain error.

Calculate the number of LSBs equivalent to the gain error

and subtract this number from the default value of the M

Calibration Example

Nominal offset coefficient = 32,768

Nominal gain coefficient = 65,535

For example, the gain error = 0.5%, and the offset error = 100 mV.

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