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| 型号： | EVAL-AD5522EBDZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 30/64页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR 14X14MM AD5522 |
| 标准包装： | 1 |
| 主要目的： | 测试与测量，参数测量单元（PMU） |
| 已用 IC / 零件： | AD5522 |
| 已供物品： | 板 |
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AD5522
Data Sheet
Rev. E | Page 36 of 64
DAC LEVELS
Each channel contains five dedicated DAC levels: one for the
force amplifier, one each for the clamp high and clamp low levels,
and one each for the comparator high and comparator 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 which node on the string the voltage is tapped off before
being fed into the output amplifier.
The transfer function for DAC outputs is as follows:
VOUT = 4.5 × VREF × (X2/216) (3.5 × VREF ×
(OFFSET_DAC_CODE/216)) + DUTGND
where:
VREF is the reference voltage and is in the range of 2 V to 5 V.
X2 is the calculated DAC code value and is in the range of 0 to
65,535 (see the Gain and Offset Registers section). OFFSET_DAC_
CODE is the code loaded to the offset DAC. It is multiplied by
3.5 in the transfer function. On power-up, the default code
loaded to the offset DAC is 0xA492; with a 5 V reference, this
gives a span of ±11.25 V.
OFFSET DAC
The AD5522 is capable of forcing a 22.5 V (4.5 × VREF) voltage
span. Included on chip is one 16-bit offset DAC (one for all four
channels) that allows for adjustment of the voltage range.
The usable range is 16.25 V to +22.5 V. Zero scale loaded to
the offset DAC gives a full-scale range of 0 V to 22.5 V, midscale
gives ±11.25 V, and the most useful negative range is 16.25 V
to +6.25 V. Full scale loaded to the offset DAC does not give a
useful output voltage range, because the output amplifiers are
limited by the available footroom. Table 13 shows the effect of
the offset DAC on the other DACs in the device.
Table 13. Relationship of Offset DAC to Other DACs
(VREF = 5 V)
Offset DAC Code
DAC Code
DAC Output Voltage (V)
0
32,768
+11.25
65,535
+22.50
32,768
0
8.75
32,768
+2.50
65,535
+13.75
42,130
0
11.25
32,768
0
65,535
+11.25
60,855
0
16.25
32,768
5.00
65,535
+6.25
65,535
Footroom limitations
The power supplies should be selected to support the required
range and should take into account amplifier headroom and
footroom and sense resistor voltage drop (±4 V).
Therefore, depending on the headroom available, the input to
the force amplifier can be unipolar positive or bipolar, either
symmetrical or asymmetrical about DUTGND, but always
within a voltage span of 22.5 V.
The offset DAC offsets all DAC functions. It also centers the
current range so that zero current always flows at midscale
code, regardless of the offset DAC setting.
Rearranging the transfer function for the DAC output gives the
following equation to determine which offset DAC code is
required for a given reference and output voltage range.
OFFSET_DAC_CODE = (216 × (VOUT DUTGND))/
(3.5 × VREF) ((4.5 × DAC_CODE)/3.5)
When the output range is adjusted by changing the default value
of the offset DAC, an extra offset is introduced due to the gain
error of the offset DAC channel. The amount of offset is depen-
dent on the magnitude of the reference and how much the
offset DAC channel deviates from its default value. See the
Specifications section for this offset. The worst-case offset
occurs when the offset DAC channel is at positive or negative
full scale. This value can be added to the offset present in the
main DAC channel to give an indication of the overall offset
for that channel. In most cases, the offset can be removed by
programming the C register of the channel with an appropriate
value. The extra offset caused by the offset DAC needs to be
taken into account only when the offset DAC is changed from
its default value.
GAIN AND OFFSET REGISTERS
Each DAC level has an independent gain (M) register and an
independent offset (C) register, which allow trimming out of
the gain and offset errors of the entire signal chain, including
the DAC. All registers in the AD5522 are volatile, so they must
be loaded on power-on during a calibration cycle. Data from
the X1 register is operated on by a digital multiplier and adder
controlled by the contents of the M and C registers. The cali-
brated DAC data is then stored in the X2 register.
The digital input transfer function for each DAC can be
represented as follows:
X2 = [(M + 1)/2n × X1] + (C 2n 1)
where:
X2 is the data-word loaded to the resistor-string DAC.
X1 is the 16-bit data-word written to the DAC input register.
M is the code in the gain register (default code = 216 1). The M
register is 15 bits (D15 to D1, the LSB is a don’t care).
C is the code in the offset register (default code = 215).
n is the DAC resolution (n = 16).
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