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參數(shù)資料
| 型號(hào)： | ADC12762 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 12-Bit, 1.4 MHz, 300 mW A/D Converter with Input Multiplexer and Sample/Hold |
| 中文描述： | 12位，1.4兆赫，300毫瓦的A / D輸入多路復(fù)用器和轉(zhuǎn)換器采樣/保持 |
| 文件頁數(shù)： | 11/20頁 |
| 文件大?。?/td> | 427K |
| 代理商： | ADC12762 |
Functional Description
(Continued)
The resistor string near the center of the block diagram in
Figure 4generates the 6-bit and 10-bit reference voltages for
the first two conversions. Each of the 16 resistors at the bot-
tom of the string is equal to 1/1024 of the total string resis-
tance. These resistors form the
LSB Ladder
(The weight of
each resistor on the LSB ladder is actually equivalent to four
12-bit LSBs. It is called the LSB ladder because it has the
highest resolution of all the ladders in the converter) and
have a voltage drop of 1/1024 of the total reference voltage
(V
V
REF
) across each of them. The remaining resis-
tors form the
. It is comprised of eight groups of
eight resistors each connected in series (the lowest MSB
ladder resistor is actually the entire LSB ladder). Each MSB
Ladder section has
1
8
of the total reference voltage across it.
Within a given MSB ladder section, each of the eight MSB
resistors has
1
6
4 of the total reference voltage across it. Tap
points are found between all of the resistors in both the MSB
and LSB ladders. The Comparator MultipIexer can connect
any of these tap points, in two adjacent groups of eight, to
the sixteen comparators shown at the right of Figure 4 This
function provides the necessary reference voltages to the
comparators during the first two flash conversions.
The six comparators, seven-resistor string (Estimator DAC
ladder), and Estimator Decoder at the left of Figure 4 form
the Voltage Estimator. The Estimator DAC, connected be-
tween V
and V
, generates the reference voltages
for the six Voltage Estimator comparators. The comparators
perform a very low resoIution A/D conversion to obtain an
“estimate” of the input voltage. This estimate is used to con-
trol the placement of the Comparator Multiplexer, connecting
the appropriate MSB ladder section to the sixteen flash com-
parators. A total of only 22 comparators (6 in the Voltage Es-
timator and 16 in the flash converter) is required to quantize
the input to 6 bits, instead of the 64 that would be required
using a traditional 6-bit flash.
Prior to a conversion, the Sample/Hold switch is closed, al-
lowing the voltage on the S/H capacitor to track the input
voItage. Switch 1 is in position 1. A conversion begins by
opening the Sample/Hold switch and latching the output of
the Voltage Estimator. The estimator decoder then selects
two adjacent banks of tap points aIong the MSB ladder.
These sixteen tap points are then connected to the sixteen
flash converters. For exampIe, if the input voltage is between
5/16 and 7/16 of V
(V
=V
V
), the estimator
decoder instructs the comparator multiplexer to select the
sixteen tap points between 2/8 and 4/8 (4/16 and 8/16) of
V
and connects them to the sixteen flash converters. The
first flash conversion is now performed, producing the first 6
MSBs of data.
At this point, Voltage Estimator errors as large as 1/16 of
V
will be corrected since the flash converters are con-
nected to ladder voltages that extend beyond the range
specified by the Voltage Estimator. For example, if (7/
16)V
<
V
<
(9/16)V
, the Voltage Estimator’s com-
parators tied to the tap points below (9/16)V
will output
“1”s (000111). This is decoded by the estimator decoder to
“10”. The 16 comparators will be placed on the MSB ladder
tap points between (
3
8
)V
and (
5
8
)V
. This overlap of
(1/16)V
REF
will automatically cancel a Voltage Estimator er-
ror of up to 256 LSBs. If the first flash conversion determines
that the input voltage is between (
3
8
)V
and ((4/8)V
LSB/2), the Voltage Estimator’s output code will be corrected
by subtracting “1”, resulting in a corrected value of “01” for
the first two MSBs. If the first flash conversion determines
that the input voltage is between (4/8)V
LSB/2) and
(
5
8
)V
REF
, the voltage estimator’s output code is unchanged.
The results of the first flash and the Voltage Estimator’s out-
put are given to the factory-programmed on-chip EEPROM
which returns a correction code corresponding to the error of
the MSB ladder at that tap. This code is converted to a volt-
age by the Correction DAC. To generate the next four bits,
SW1 is moved to position 2, so the ladder voltage and the
correction voltage are subtracted from the input voltage. The
remainder is applied to the sixteen flash converters and
compared with the 16 tap points from the LSB ladder.
The result of this second conversion is accurate to 10 bits
and describes the input remainder as a voltage between two
tap points (V
and V
) on the LSB ladder. To resolve the last
two bits, the voltage across the ladder resistor (between V
H
and V
) is divided up into 4 equal parts by the capacitive volt-
age divider, shown in Figure 5 The divider also creates 6
LSBs below V
and 6 LSBs above V
to provide overlap
used by the digital error correction. SW1 is moved to position
3, and the remainder is compared with these 16 new volt-
ages. The output is combined with the results of the Voltage
Estimator, first flash, and second flash to yield the final 12-bit
result.
By using the same sixteen comparators for all three flash
conversions, the number of comparators needed by the
multi-step converter is significantly reduced when compared
to standard multi-step techniques.
Applications Information
MODES OF OPERATION
The ADC12762 has two interface modes: An interrupt/read
mode and a high speed mode. Figure 1 and Figure 2 show
the timing diagrams for these interfaces.
In order to clearly show the relationship between S/H, CS,
RD, and OE, the control logic decoding section of the
ADC12762 is shown in Figure 6
Interrupt Interface
As shown in Figure 1 the falling edge of S/H holds the input
voltage and initiates a conversion. At the end of the conver-
sion, the EOC output goes high and the INT output goes low,
indicating that the conversion results are latched and may be
read by pulling RD low. The falling edge of RD resets the INT
line. Note that CS must be low to enable S/H or RD.
High Speed Interface
The Interrupt interface works well at lower speeds, but few
microprocessors could keep up with the 1 μs interrupts that
would be generated if the ADC12762 was running at full
speed. The most efficient interface is shown in Figure 2
Here the output data is always present on the databus, and
the INT to RD delay is eliminated.
www.national.com
11
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