AD5370BSTZ datasheet(15/24 Pages) AD

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Preliminary Technical Data

AD5370

Rev. P

rF | Page 15 of 24

12.36V + 0.14V = 12.5V

VREF Calculation

Actual Output Range = 12.5V, that is -4.25V to +8.25V

(centered);

VREF = (8.25V + 4.25V)/4 = 3.125V

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 reference level but may reduce

the performance by overcompaction of the transfer

function.

Use a combination of these two approaches

CALIBRATION

The user can perform a system calibration on the AD5370 to

reduce gain and offset errors to below 1 LSB. This is achieved

by calculating new values for the M and C registers and reprogram-

ming them.

Reducing Zero-scale and Full-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 with the

required value. This gives the zero-scale error.

Calculate the number of LSBs equivalent to the

error and subtract this from the default value of

the C register. Note that only negative zero-scale error can

be reduced.

Full-scale 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 with the

required value. Add this error to the zero-scale error. This

is the full-scale error.

Calculate the number of LSBs equivalent to the full-scale

error and subtract it from the default value of the M

reduced.

The M and C registers should not be programmed until

both zero-scale and full-scale errors have been calculated.

AD5370 Calibration Example

This example assumes that a −4 V to +8 V output is required.

The DAC output is set to −4 V but measured at −4.03 V. This

gives an zero-scale error of −30 mV.

1 LSB = 12 V/65536 = 183.105 µV

30 mV = 164 LSB

164 LSB should be added to the default C register value:

(32768 + 164) = 32932

32932 should be programmed to the C register

The full-scale error can now be removed. The output is set to +8

V and a value of +8.02 V is measured. The full-scale error is

+20 mV – (–30 mV) = +50 mV

This is a full-scale error of +50 mV.

50 mV = 273 LSBs

273 LSB should be subtracted from the default M register

value: (65535 − 273) = 65262

65262 should be programmed to the M register

ADDITIONAL CALIBRATION

The techniques described in the previous text are usually

enough to reduce the zero-scale and full-scale errors in most

applications. However, there are limitations whereby the errors

may not be sufficiently removed. For example, the offset (C)

negative zero-scale error. A positive offset cannot be reduced.

Likewise, if the maximum voltage is below the ideal value, i.e. a

negative full-scale error, the gain (M) register cannot be used to

increase the gain to compensate for the error.

These limitations can be overcome by increasing the reference

value. With a 3V reference a 12V span will be achieved. The

ideal voltage range, for the AD5371, would be −4V to +8V.

Using a 3.1V reference would increase the range to −4.133V to

+8.2667V. Clearly, in this case, the offset and gain errors are

insignificant and the M and C registers can be used to raise the

negative voltage to −4V and then reduce the maximum voltage

down to +8V to give the most accurate values possible.

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