AD5560JSVUZ datasheet(42/67 Pages) AD

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Data Sheet

AD5560

Rev. E | Page 41 of 66

REFERENCE SELECTION

The voltage applied to the VREF pin determines the output

voltage range and span applied to the force amplifier, clamp,

and comparator inputs and the current ranges.

This device can be used with a reference input ranging from

2 V to 5 V. However, for most applications, a reference input

of 5 V is able to meet all voltage range requirements. The DAC

amplifier gain is 5.125, which gives a DAC output span of

25.625 V. The DACs have gain and offset registers that can

be used to calibrate out system errors. In addition, the gain

desired force voltage range.

Using a 5 V reference and setting the m (gain) register to one-

fourth scale or 0x4000 gives an output voltage span of 6.25 V.

Because the force DAC has 18 bits of resolution even with only

one-fourth of the output voltage span, it is still possible to

achieve 16-bit resolution in this 6.25 V range.

The measure current amplifier has two gain settings, 10 and 20.

The two gain settings allow users to achieve the quoted/speci-

fied current ranges with large or small voltage swings. The 20

gain setting is intended for use with a 5 V reference, and the 10

gain setting is for use with a 2.5 V reference. Both combinations

ensure the specified current ranges. Other VREF/gain setting

combinations should be used only to achieve smaller current

ranges. See Table 27 for suggested references for use with the

AD5560.

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.

Reducing Zero-Scale Error

Zero-scale error can be reduced as follows:

1. Set the output to the lowest possible value.

2. Measure the actual output voltage and compare it to the

required value. This is the zero-scale error.

3. Calculate the number of LSBs equivalent to the zero-scale

error, and add or subtract this number to the default value

of the c register.

Reducing Gain Error

Gain error can be reduced as follows:

1. Measure the zero-scale error.

2. Set the output to the highest possible value.

3. Measure the actual output voltage and compare it to the

required value. This is the gain error.

4. 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 (0x8000)

Nominal gain coefficient = 65,535 (0xFFFF)

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

Gain error (0.5%) calibration is as follows:

65,535 × 0.995 = 65,207

Therefore, load Code 1111 1110 1011 0111 (0xFEB7) to the

m register.

Offset error (100 mV) calibration is as follows:

LSB size = 10.25/65,535 = 156 µV

Offset coefficient for 100 mV offset = 100/0.156 = 641 LSBs

Therefore, load Code 0111 1101 0111 1111 (0x7D7F) to the

c register.

ADDITIONAL CALIBRATION

The techniques described in the Calibration section are usually

sufficient to reduce the zero-scale and gain errors. 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 negative zero-scale error. A positive

offset cannot be reduced. Likewise, if the maximum voltage is

below the ideal value, that is, a negative gain error, the gain (m)

the error. These limitations can be overcome by increasing the

reference value.

SYSTEM LEVEL CALIBRATION

There are many ways to calibrate the device on power-on.

Following is an example of how to calibrate the FIN DAC

registers (Register 0x8 to Register 0xA) of the device without a

DUT or DUT board connected. The calibration procedure for

the force and measure circuitry is as follows:

1. Calibrate the force voltage (two-point calibration).

a. Write zero scale to the FIN DAC registers

(Register 0x8 to Register 0xA).

b. Connect SYS_FORCE to FORCE (via SW8) and

SYS_SENSE to SENSE (via SW9), and close the internal

force/sense switch (SW11).

c. Using the system PMU, measure the error between the

voltage at FORCE/SENSE and the desired value.

d. Similarly, load full scale to the FIN DAC registers

(Register 0x8 to Register 0xA) and measure the error

between the voltage at FORCE/SENSE and the desired

value.

e. Calculate the m and c values.

Load these values to the appropriate FIN DAC m and

FIN DAC c registers (Register 0x9 and Register 0xA).

2. Calibrate the measure voltage (two-point calibration).

a. Connect SYS_FORCE to FORCE (via SW8) and

SYS_SENSE to SENSE (via SW9), and close the

internal force/sense switch (via SW11).

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