AD5372BCPZ datasheet(14/25 Pages) AD

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AD5372/AD5373

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 (R1 and R2 are very much greater

than R6). 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

The output voltage of a DAC in the AD5372/AD5373 is

dependent on the value in the input register, the value of the M

and C registers, and the offset from the Offset DAC. The

transfer functions for the AD5372 and AD5373 are shown

below.

AD5372 Transfer Function

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 )/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 = 21844

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

c = code in offset register - default code = 214.

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

multiplied by 4 in the transfer function as this DAC is a 14 bit

device. On power up the default code loaded to the offset DAC

is 5461 (0x1555). With a 3V reference this gives a span of -4 V

to +8 V.

AD5373 Transfer Function

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 = 5461

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

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

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

multiplied by 4 in the transfer function as this DAC is a 14 bit

device. On power up the default code loaded to the offset DAC

is 5461 (0x1555). With a 3V reference this gives a span of -4 V

to +8 V.

REFERENCE SELECTION

The AD5372/AD5373 has two reference input pins. The voltage

applied to the reference pins determines the output voltage span

on VOUT0 to VOUT31. VREF0 determines the voltage span for

VOUT0 to VOUT7 (Group 0) and VREF1 determines the

voltage span for VOUT8 to VOUT31 (Group 1 to Group 3).

The reference voltage applied to each VREF pin can be

different, if required, allowing the groups to have a different

voltage spans. The output voltage range can be adjusted further

by programming the offset and gain registers for each channel

as well as programming the offset DACs. 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 AD5372/AD5373 are 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

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