参数资料
型号: AD797AR-REEL7
厂商: Analog Devices Inc
文件页数: 4/20页
文件大小: 0K
描述: IC OPAMP GP 110MHZ ULDIST 8SOIC
标准包装: 750
放大器类型: 通用
电路数: 1
转换速率: 20 V/µs
增益带宽积: 110MHz
-3db带宽: 8MHz
电流 - 输入偏压: 250nA
电压 - 输入偏移: 25µV
电流 - 电源: 8.2mA
电流 - 输出 / 通道: 50mA
电压 - 电源,单路/双路(±): ±5 V ~ 15 V
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 8-SOIC(0.154",3.90mm 宽)
供应商设备封装: 8-SO
包装: 带卷 (TR)
AD797
Data Sheet
Rev. J | Page 12 of 20
NOISE AND SOURCE IMPEDANCE CONSIDERATIONS
The AD797 ultralow voltage noise of 0.9 nV/√Hz is achieved
with special input transistors running at nearly 1 mA of collector
current. Therefore, it is important to consider the total input-
referred noise (eNtotal), which includes contributions from voltage
noise (eN), current noise (iN), and resistor noise (√4 kTRS).
2
/
1
2
]
)
(
4
[
S
N
S
N
R
i
kTR
e
total
e
×
+
=
(1)
where RS is the total input source resistance.
This equation is plotted for the AD797 in Figure 34. Because
optimum dc performance is obtained with matched source
resistances, this case is considered even though it is clear from
Equation 1 that eliminating the balancing source resistance
lowers the total noise by reducing the total RS by a factor of 2.
At very low source resistance (RS < 50 Ω), the voltage noise of the
amplifier dominates. As source resistance increases, the Johnson
noise of RS dominates until a higher resistance of RS > 2 kΩ is
achieved; the current noise component is larger than the
resistor noise.
00846-
033
100
1
0.1
10
100
1000
10000
SOURCE RESIST
ANCE ()
N
OIS
E(
nV
/√
H
z)
TOTAL NOISE
RESISTOR
NOISE
ONLY
Figure 34. Noise vs. Source Resistance
The AD797 is the optimum choice for low noise performance if
the source resistance is kept <1 kΩ. At higher values of source
resistance, optimum performance with respect to only noise is
obtained with other amplifiers from Analog Devices (Table 5).
For up to date information, see AN-940 at www.analog.com.
Table 5. Recommended Amplifiers for Different Source
Impedances
RS (k)
Recommended Amplifier
0 to <1
1 to <10
10 to <100
>100
LOW FREQUENCY NOISE
Analog Devices specifies low frequency noise as a peak-to-peak
quantity in a 0.1 Hz to 10 Hz bandwidth. Several techniques can
be used to make this measurement. The usual technique involves
amplifying, filtering, and measuring the amplifier noise for a
predetermined test time. The noise bandwidth of the filter is
corrected for, and the test time is carefully controlled because
the measurement time acts as an additional low frequency roll-off.
The plot in Figure 6 uses a slightly different technique: an FFT-
based instrument (Figure 35) is used to generate a 10 Hz brickwall
filter. A low frequency pole at 0.1 Hz is generated with an external
ac coupling capacitor, which is also the instrument being dc coupled.
Several precautions are necessary to attain optimum low
frequency noise performance:
Care must be used to account for the effects of RS. Even
a 10 Ω resistor has 0.4 nV/√Hz of noise (an error of 9%
when root sum squared with 0.9 nV/√Hz).
The test setup must be fully warmed up to prevent eOS drift
from erroneously contributing to input noise.
Circuitry must be shielded from air currents. Heat flow out
of the package through its leads creates the opportunity for
a thermoelectric potential at every junction of different metals.
Selective heating and cooling of these by random air currents
appears as 1/f noise and obscures the true device noise.
The results must be interpreted using valid statistical
techniques.
7
4
6
2
3
HP 3465
DYNAMIC SIGNAL
ANALYZER
(10Hz)
1
100k
*
VOUT
+VS
–VS
1.5F
AD797
00846-
034
*USE THE POWER SUPPLY BYPASSING SHOWN IN FIGURE 35.
Figure 35. Test Setup for Measuring 0.1 Hz to 10 Hz Noise
WIDEBAND NOISE
Due to its single-stage design, the noise of the AD797 is flat
over frequencies from less than 10 Hz to beyond 1 MHz. This
is not true of most dc precision amplifiers, where second-stage
noise contributes to input-referred noise beyond the audio
frequency range. The AD797 offers new levels of performance in
wideband imaging applications. In sampled data systems, where
aliasing of out-of-band noise into the signal band is a problem,
the AD797 outperforms all previously available IC op amps.
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