AD5363 Analog Devices, AD5363 Datasheet - Page 19

• Manufacturer Part Number: AD5363
• Description: 8-Channel, 14-Bit, Serial Input, Voltage-Output DAC
• Manufacturer: Analog Devices

Specifications of AD5363

• Resolution (bits): 14bit
• Dac Settling Time: 20µs
• Single-supply: No
• Dac Type: Voltage Out
• Dac Input Format: Ser,SPI

Reference Selection Example
If
Nominal output range = 20 V (−10 V to +10 V)
Offset error = ±100 mV
Gain error = ±3%, and
SIGGND = AGND = 0 V
Then
Gain error = ±3%
Offset error = ±100 mV
VREF calculation
If the solution yields an inconvenient reference level, the user
can adopt one of the following approaches:
•
•
•
CALIBRATION
The user can perform a system calibration on the AD5362/
AD5363 to reduce gain and offset errors to below 1 LSB. This
reduction is achieved by calculating new values for the M and
C registers and reprogramming them.
The M and C registers should not be programmed until both
the zero-scale and full-scale errors are calculated.
Reducing Zero-Scale Error
Zero-scale error can be reduced as follows:
1.
2.
3.
=> Maximum positive gain error = 3%
=> Output range including gain error = 20 + 0.03(20) = 20.6 V
=> Maximum offset error span = 2(100 mV) = 0.2 V
=> Output range including gain error and offset error =
20.6 V + 0.2 V = 20.8 V
Actual output range = 20.6 V, that is, −10.3 V to +10.3 V
(centered);
VREF = (10.3 V + 10.3 V)/4 = 5.15 V
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 can reduce the performance by overcompaction of
the transfer function.
Use a combination of these two approaches.
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 error and
add this number to the default value of the C register. Note
that only negative zero-scale error can be reduced.
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Reducing Full-Scale Error
Full-scale error can be reduced as follows:
1.
2.
3.
4.
AD5362 Calibration Example
This example assumes that a −10 V to +10 V output is required.
The DAC output is set to −10 V but measured at −10.03 V. This
gives a zero-scale error of −30 mV.
The full-scale error can now be calculated. The output is set to
10 V and a value of 10.02 V is measured. This gives a full-scale
error of +20 mV and a span error of +20 mV – (–30 mV) =
+50 mV.
The errors can now be removed as follows:
1.
2.
3.
ADDITIONAL CALIBRATION
The techniques described in the previous section 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 reduced. For example, the offset (C)
register can only be used to reduce the offset caused by the
negative zero-scale error. A positive offset cannot be reduced.
Likewise, if the maximum voltage is below the ideal value, that
is, 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 2.5 V reference, a 10 V span is achieved. The ideal
voltage range, for the AD5362 or the AD5363, is −5 V to +5 V.
Using a +2.6 V reference increases the range to −5.2 V to +5.2 V.
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 −5 V and then reduce the maximum voltage to +5 V
to give the most accurate values possible.
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. Add this error to the zero-scale error. This
is the span error, which includes the full-scale error.
Calculate the number of LSBs equivalent to the span error
and subtract this number from the default value of the M
register. Note that only positive full-scale error can be
reduced.
1 LSB = 20 V/65,536 = 305.176 μV
30 mV = 98 LSBs
50 mV = 164 LSBs
Add 98 LSBs to the default C register value:
(32,768 + 98) = 32,866
Subtract 164 LSBs from the default M register value:
(65,535 − 164) = 65,371
Program the M register to 65,371; program the C register
to 32,866.
AD5362/AD5363