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Preliminary Technical Data

AD5371

Rev. P

rF | Page 15 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 11. Output Amplifier and Offset DAC

Figure 11 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

From the previous text, it can be seen that the output voltage of

a DAC in the AD5371 depends on the value in the input

the Offset DAC. The transfer function is given by:

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 AD5371 has three 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 VOUT15 (Group 1) and VREF2

determines the voltage span for VOUT16 to VOUT31 (Group 2

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 and span 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 AD5371 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

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

values. Note that VDD and VSS must provide sufficient

headroom.

Calculate the value of VREF as follows:

VREF = (VOUTMAX – VOUTMIN)/4

Reference Selection Example

Nominal Output Range = 12V (-4V to +8V)

Offset Error = ±70mV

Gain Error = ±3%

SIGGND = AGND = 0V

Gain Error = ±3%

=> Maximum Positive Gain Error = +3%

=> Output Range incl. Gain Error = 12 + 0.03(12)=12.36V

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