AD5370BSTZ-REEL View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AD5370BSTZ-REEL Datasheet PDF : 28 Pages
| |||
AD5370
OUTPUT AMPLIFIER
The output amplifiers can swing to 1.4 V below the positive
supply and 1.4 V above the negative supply, which 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.
DAC
CHANNEL
R5
S2
60kΩ
R1
20kΩ CLR
S1
CLR
VOUT
R6
10kΩ
CLR
S3
R4
R3
60kΩ 20kΩ
R2
20kΩ
SIGGND
SIGGND
OFFSET
DAC
Figure 21. Output Amplifier and Offset DAC
Figure 21 shows details of a DAC output amplifier and its
connections to its corresponding offset DAC. On power-up, S1
is open, disconnecting the amplifier from the output. S3 is
closed; thus, the output is pulled to the corresponding SIGGND
(R1 and R2 are 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 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 the
previously described condition until VDD > 6 V and VSS
< −4 V and the initialization sequence has finished. The outputs
then go to their power-on default values.
TRANSFER FUNCTION
OUTPUT
VOLTAGE
8V
ACTUAL
TRANSFER
FUNCTION
FULL-SCALE
ERROR
+
ZERO-SCALE
ERROR
IDEAL
TRANSFER
FUNCTION
0
DAC CODE
16383
ZERO-SCALE
–4V
ERROR
Figure 22. DAC Transfer Function
The output voltage of a DAC in the AD5370 is dependent on the
value in the input register, the value of the M and C registers,
and the value in the offset DAC. The transfer functions for the
AD5370 are shown in the following section.
The input code is the value in the X1A or X1B register that is
applied to DAC (X1A, X1B default code = 5461), as follows:
DAC
_
CODE
=
INPUT
_
CODE × (M
216
+1)
+
C
−
215
DAC output voltage is calculated as follows:
VOUT = 4 ×VREF×
DAC_CODE
−
(4
× OFFSET
216
_
CODE
)
+
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.
M = code in gain register − default code = 216 – 1.
C = code in offset register − default code = 215.
OFFSET_CODE is the code loaded to the offset DAC. It is
multiplied by 4 in the transfer function because the offset DAC
is a 14-bit device. On power-up, the default code loaded to the
offset DAC is 5461 (0x1555). With a 3 V reference, this gives a
span of −4 V to +8 V.
REFERENCE SELECTION
The AD5370 has two reference input pins. The voltage applied
to the reference pins determines the output voltage span on
VOUT0 to VOUT39. VREF0 determines the voltage span for
VOUT0 to VOUT7 (Group 0) and VREF1 determines the
voltage span for VOUT8 to VOUT39 (Group 2 to Group 4).
The reference voltage applied to each VREF pin can be
different, if required, allowing each group 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 and by programming the offset DAC channels. 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 AD5370 are used, 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:
1. Identify the nominal output range on VOUT.
2. Identify the maximum offset span and the maximum gain
required on the full output signal range.
3. Calculate the new maximum output range on VOUT,
including the expected maximum offset and gain errors.
4. 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.
5. Calculate the value of VREF as follows:
VREF = (VOUTMAX − VOUTMIN)/4
Rev. 0 | Page 17 of 28
Share Link: 