AD5362 datasheet(19/29 Pages) AD

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AD5362/AD5363

Rev. A | Page 18 of 28

OUTPUT AMPLIFIER

Because the output amplifiers can swing to 1.4 V below the

positive supply and 1.4 V above the negative supply, this limits

how much the output can be offset for a given reference voltage.

For example, it is not possible to have a unipolar output range

of 20 V, because the maximum supply voltage is ±16.5 V.

CLR

DAC

CHANNEL

OFFSET

DAC

OUTPUT

10kΩ

20kΩ

60kΩ

20kΩ

SIGGNDx

60kΩ

20kΩ

Figure 23. Output Amplifier and Offset DAC

Figure 23 shows details of a DAC output amplifier and its connec-

tions to the offset DAC. On power-up, S1 is open, disconnecting

the amplifier from the output. S3 is closed, so the output is pulled

to SIGGNDx (R1 and R2 are greater than R6). S2 is also closed to

prevent the output amplifier from being open-loop. If CLR is low at

power-up, the output remains in this condition until CLR is taken

high. The DAC registers can be programmed, and the outputs

assume the programmed values when CLR is taken high. Even if

CLR is high at power-up, the output remains in this condition

until VDD > 6 V and VSS < −4 V and the initialization sequence has

finished. The outputs then go to their power-on default value.

TRANSFER FUNCTION

The output voltage of a DAC in the AD5362/AD5363 is depen-

dent on the value in the input register, the value of the M and C

registers, and the value in the offset DAC.

AD5362 Transfer Function

The input code is the value in the X1A or X1B register that is

applied to the DAC (X1A, X1B default code = 32,768).

DAC_CODE = INPUT_CODE × (M + 1)/216 + C − 215

where:

M = code in gain register − default code = 216 – 1.

C = code in offset register − default code = 215.

The DAC output voltage is calculated as follows:

VOUT = 4 × VREF × (DAC_CODE − (OFFSET_CODE ×

4))/216 + VSIGGND

where:

DAC_CODE should be within the range of 0 to 65,535.

For 12 V span, VREF = 3.0 V.

For 20 V span, VREF = 5.0 V.

OFFSET_CODE is the code loaded to the offset DAC. It is

multiplied by 4 in the transfer function because this DAC is a

14-bit device. On power-up, the default code loaded to the

offset DAC is 8192 (0x2000). With a 5 V reference, this gives

a span of −10 V to +10 V.

AD5363 Transfer Function

The input code is the value in the X1A or X1B register that is

applied to the DAC (X1A, X1B default code = 8192).

DAC_CODE = INPUT_CODE × (M + 1)/214 + C − 213

where:

M = code in gain register − default code = 214 – 1.

C = code in offset register − default code = 213.

The DAC output voltage is calculated as follows:

VOUT = 4 × VREF × (DAC_CODE − OFFSET_CODE)/

214 + VSIGGND

where:

DAC_CODE should be within the range of 0 to 16,383.

For 12 V span, VREF = 3.0 V.

For 20 V span, VREF = 5.0 V.

OFFSET_CODE is the code loaded to the offset DAC. On power-

up, the default code loaded to the offset DAC is 8192 (0x2000).

With a 5 V reference, this gives a span of −10 V to +10 V.

REFERENCE SELECTION

The AD5362/AD5363 have two reference input pins. The

voltage applied to the reference pins determines the output

voltage span on VOUT0 to VOUT7. VREF0 determines the

voltage span for VOUT0 to VOUT3 (Group 0), and VREF1

determines the voltage span for VOUT4 to VOUT7 (Group 1).

The reference voltage applied to each VREF pin can be differ-

ent, if required, allowing each group of four channels to have a

different voltage span. The output voltage range and span can

be adjusted further by programming the offset and gain

registers for each channel as well as programming the offset

DAC. If the offset and gain features are not used (that is, the M

and C registers are left at their default values), the required

reference levels can be calculated as follows:

VREF = (VOUTMAX − VOUTMIN)/4

If the offset and gain features of the AD5362/AD5363 are used,

the required output range is slightly different. The selected

output range should take into account the system offset and

gain errors that need to be trimmed out. Therefore, the selected

output range should be larger than the actual, required range.

The required reference levels can be calculated as follows:

Identify the nominal output range on VOUT.

Identify the maximum offset span and the maximum gain

required on the full output signal range.

Calculate the new maximum output range on VOUT,

including the expected maximum offset and gain errors.

Choose the new required VOUTMAX and VOUTMIN, keep-

ing the VOUT limits centered on the nominal values. Note

that VDD and VSS must provide sufficient headroom.

Calculate the value of VREF as follows:

VREF = (VOUTMAX − VOUTMIN)/4

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