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參數(shù)資料
| 型號： | EVAL-AD7608EDZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 11/32頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR AD7608 |
| 標準包裝： | 1 |
| ADC 的數(shù)量： | 8 |
| 位數(shù)： | 18 |
| 采樣率（每秒）： | 200k |
| 數(shù)據(jù)接口： | DSP，MICROWIRE?，并聯(lián)，QSPI?，串行，SPI? |
| 輸入范圍： | ±10 V |
| 在以下條件下的電源（標準）： | 100mW @ 200kSPS |
| 工作溫度： | -40°C ~ 85°C |
| 已用 IC / 零件： | AD7608 |
| 已供物品： | 板 |
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Data Sheet
AD7608
Rev. A | Page 19 of 32
THEORY OF OPERATION
CONVERTER DETAILS
The AD7608 is a data acquisition system that employs a high
speed, low power, charge redistribution, successive approxi-
mation analog-to-digital converter (ADC) and allows the
simultaneous sampling of eight analog input channels. The
analog inputs on the AD7608 can accept true bipolar input
signals. The RANGE pin is used to select either ±10 V or
±5 V as the input range. The AD7608 operates from a single
5 V supply.
The AD7608 contains input clamp protection, input signal
scaling amplifiers, a second-order antialiasing filter, track-and-
hold amplifiers, an on-chip reference, reference buffers, a high
speed ADC, a digital filter, and high speed parallel and serial
interfaces. Sampling on the AD7608 is controlled using the
CONVST x signals.
ANALOG INPUT
Analog Input Ranges
The AD7608 can handle true bipolar, single-ended input voltages.
The logic level on the RANGE pin determines the analog input
range of all analog input channels. If this pin is tied to a logic
high, the analog input range is ±10 V for all channels. If this pin
is tied to a logic low, the analog input range is ±5 V for all
channels. A logic change on the RANGE pin has an immediate
effect on the analog input range; how-ever, there is typically
a settling time of approximately 80 s, in addition to the normal
acquisition time requirement. The recommended practice is to
hardwire the RANGE pin according to the desired input range
for the system signals.
During normal operation, the applied analog input voltage
should remain within the analog input range selected via the
range pin. A RESET pulse must be applied after power-up to
ensure the analog input channels are configured for the range
selected.
When in a power-down mode, it is recommended to tie the
analog inputs to GND. As per the input clamp protection section,
the overvoltage clamp protection is recommended for use in
transient overvoltage conditions and should not remain active
for extended periods. Stressing the analog inputs outside of the
conditions mentioned here may degrade the Bipolar Zero Code
error and THD performance of the AD7608.
Analog Input Impedance
The analog input impedance of the AD7608 is 1 M. This is
a fixed input impedance that does not vary with the AD7608
sampling frequency. This high analog input impedance elimi-
nates the need for a driver amplifier in front of the AD7608,
allowing for direct connection to the source or sensor. With
the need for a driver amplifier eliminated, bipolar supplies
(which are often a source of noise in a system) can be
removed from the signal chain.
Analog Input Clamp Protection
Figure 31 shows the analog input structure of the AD7608. Each
AD7608 analog input contains clamp protection circuitry. Despite
single 5 V supply operation, this analog input clamp protection
allows for an input overvoltage up to ±16.5 V.
1M
CLAMP
Vx
1M
CLAMP
VxGND
SECOND-
ORDER
LPF
RFB
08938-
029
Figure 31. Analog Input Circuitry
Figure 32 shows the voltage vs. current characteristic of the
clamp circuit. For input voltages of up to ±16.5 V, no current
flows in the clamp circuit. For input voltages that are above
±16.5 V, the AD7608 clamp circuitry turns on.
30
–40
–30
–20
–10
0
10
20
–25
–20
–15
–10
–5
0
5
10
15
20
25
INP
UT
CL
AM
P
CURRE
NT
SOURCE VOLTAGE (V)
08938-
030
AVCC, VDRIVE = 5V
TA = 25 °C
Figure 32. Input Protection Clamp Profile
A series resistor should be placed on the analog input channels
to limit the current to ±10 mA for input voltages above ±16.5 V.
In an application where there is a series resistance on an analog
input channel, Vx, a corresponding resistance is required on the
analog input GND channel, VxGND (see Figure 33). If there is
no corresponding resistor on the VxGND channel, an offset
error occurs on that channel.
1M
CLAMP
Vx
1M
CLAMP
VxGND
RFB
C
R
ANALOG
INPUT
SIGNAL
AD7608
08938-
031
Figure 33. Input Resistance Matching on the Analog Input
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