AD5522_07 Datasheet, PDF (23/48 Pages) Analog Devices – Quad Parametric Measurement Unit With Integrated 16-Bit Level Setting DACs

Preliminary Technical Data
AD5522
DAC LEVELS
Each channel contains five dedicated DAC levels : one for the
force amplifier, one each for the clamp high and low levels and
one each for the comparator high and low levels.
The architecture of a single DAC channel consists of a 16-bit
resistor-string DAC followed by an output buffer amplifier. This
resistor-string architecture guarantees DAC monotonicity. The
16-bit binary digital code loaded to the DAC register
determines at what node on the string the voltage is tapped off
before being fed to the output amplifier.
The transfer function for DAC outputs is:
V_OUT
=
4 .5V_REF
⎜⎛
⎝
DACCODE
2¹⁶
⎟⎞
⎠
−
3.5V_REF
⎜⎛
⎝
OFFSETDAC
2¹⁶
⋅CODE
⎟⎞
⎠
+
DUTGND
Where the voltage range must be take into account the +/-4V
headroom and footroom requirements for the amplifier and
sense resistor and must be within the range -16.25V to 22.5V
(22V range + 500mV overrange to allow for correction).
OFFSET DAC
The device is capable of forcing a 22.5V (4.5 × VREF) voltage
range. Included on chip is one 16 Bit offset DAC (one for all
four channels) which allows for adjustment of the voltage range.
The useable range is -16.25V to 22.5V. Zero scale gives a full-
scale range of 0V to +22.5V, mid scale gives ±11.25V, while the
most negative useful range is in a range of -16.25V to 6.25V.
Full scale loaded to the Offset DAC does not give a useful
output voltage range as the output amplifiers are limited by
available footroom. The following table shows the effect of the
Offset DAC on the other DACs in the device.
Table 11. OFFSET DAC Relationship with other DACs with
VREF = 5V
Offset DAC
Code
0
0
0
DAC Code
0
32768
65535
DAC Output Voltage Range
0.00 V
11.25 V
22.50 V
32768
32768
32768
0
32768
65535
-8.75 V
2.50 V
13.75 V
42130
42130
42130
0
32768
65535
-11.25 V
0.00 V
11.25 V
60855
60855
60855
0
32768
65535
-16.25
-5.00
6.25
Therefore, depending on headroom available, the input to the
Force Amplifier may be unipolar positive, or bipolar, either
symmetrical or asymmetrical about DUTGND but always
within a voltage span of 22.5V.
The offset DAC offsets all DAC functions. It also centers the
current range, such that zero current always flows at midscale
code irrespective of offset DAC setting.
Rearranging the transfer function for the DAC output gives the
following equation to determine what Offset DAC code is
required for a given reference and output voltage range.
OFFSETDAC⋅CODE
=
⎜⎜⎝⎛
2¹⁶
(V_OUT − DUTGND)
3.5V_REF
⎟⎟⎠⎞
−
⎜⎛
⎝
4.5
×
DACCODE
3.5
⎟⎞
⎠
OFFSET AND GAIN REGISTERS
Each DAC level contains independent offset and gain control
registers that allow the user to digitally trim offset and gain.
These registers give the user the ability to calibrate out errors in
the complete signal chain, including the DAC, using the
internal m and c registers, which hold the correction factors. All
registers in the AD5522 are volatile, so need to be loaded on
power on during a calibration cycle.
The digital input transfer function for each DAC can be
represented as
x2 = [(m + 1)/ 2ⁿ × x1] + (c – 2^{n – 1})
where:
x2 = the data-word loaded to the resistor string DAC.
x1 = the 16-bit data-word written to the DAC input register.
m = code in gain register (default code = 2¹⁶ – 1.)
c = code in offset register (default code = 2¹⁵)
n = DAC resolution (n = 16).
The calibration engine is only engaged when data is written to
the x1 register. This has the advantage of minimizing the setup
time of the device.
CACHED X2 REGISTERS
Each DAC has a number of cached x2 values. These registers
store the result of an offset and gain calibration in advance of a
mode change. This enables the user to preload registers; allow
the calibration engine to calculate the appropriate x2 value and
store until ready to change modes. As the data is ready and held
in the appropriate register, this enables mode changing be as
time efficient as possible. If an update occurs to a DAC register
set that is currently part of the operating PMU mode, the DAC
output will update immediately (depending on LOAD
condition).
65535
-
Footroom Limitations
Rev. PrM | Page 23 of 48