AD5362BSTZ datasheet(14/25 Pages) AD

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AD5362/AD5363

Preliminary Technical Data

Rev. P

rF | Page 14 of 25

OUTPUT AMPLIFIER

As 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

20V, since the maximum supply voltage is ±16.5 V.

DAC

CHANNEL

OFFSET

DAC

CLR

SIGGND

OUTPUT

CHECK VALUE OF R1 &R5

R1,R2,R3 = 20k

Ω

R4,R5 = 60k

Ω

R6 = 10k

Ω

2049-0008

10k

Ω

Figure 10. Output Amplifier and Offset DAC

Figure 10 shows details of a DAC output amplifier and its

connections 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 SIGGND. S2 is also closed to prevent the

output amplifier being open-loop. If CLR is low at power-up,

the output will remain in this condition until CLR is taken high.

The DAC registers can be programmed, and the outputs will

assume the programmed values when CLR is taken high. Even

if CLR is high at power-up, the output will remain in the above

condition until VDD > 6 V and VSS < -4 V and the initialization

sequence has finished. The outputs will then go to their power-

on default value.

TRANSFER FUNCTION

From the foregoing, it can be seen that the output voltage of a

DAC in the AD5362/AD5363 depends on the value in the input

the Offset DAC. The transfer function is given by:

AD5362

Code applied to DAC from X1A or X1B register:-

DAC_CODE = INPUT_CODE × (m+1)/216 + c - 215

DAC output voltage:-

VOUT = 4 × VREF × (DAC_CODE – OFFSET_CODE × 4 )/216

+VSIGGND

Notes

DAC_CODE should be within the range of 0 to 65535

For 12 V span VREF = 3.0 V.

For 20 V span VREF = 5.0 V.

X1A, X1B default code = 32768

m = code in gain register; default m code = 216 – 1.

c = code in offset register; default c code = 215.

OFFSET_CODE is the 14-bit code written to the offset DAC

by 4 to make the transfer function correct, since the X, M and C

registers are 16-bit. The default value for the Offset DAC is

8192 (0x2000)

AD5363

Code applied to DAC from X1A or X1B register:-

DAC_CODE = INPUT_CODE × (m+1)/214 + c - 213

DAC output voltage:-

VOUT = 4 × VREF × (DAC_CODE – OFFSET_CODE )/214 +VSIGGND

Notes

DAC_CODE should be within the range of 0 to 16383.

For 12 V span VREF = 3.0 V.

For 20 V span VREF = 5.0 V.

X1A, X1B default code = 8192

m = code in gain register; default m code = 214 – 1.

c = code in offset register; default c code = 213.

OFFSET_CODE is the code loaded to the offset DAC. The

default value for the Offset DAC is 8192 (0x2000)

REFERENCE SELECTION

The AD5362/AD5363 has 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 and VREF1 determines the voltage span for

VOUT4to VOUT7. The reference voltage applied to each VREF

pin can be different, if required, allowing each group of 4

channels to have a different voltage span. The output voltage

range 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 (i.e. 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 AD5362AD5363are used,

then the required output range is slightly different. The chosen

output range should take into account the system offset and

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

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,

keeping the VOUT limits centered on the nominal

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