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OPA846ID 参数 Datasheet PDF下载

OPA846ID图片预览
型号: OPA846ID
PDF下载: 下载PDF文件 查看货源
内容描述: 宽带,低噪声,电压反馈运算放大器 [Wideband, Low-Noise, Voltage-Feedback OPERATIONAL AMPLIFIER]
分类和应用: 运算放大器放大器电路光电二极管
文件页数/大小: 23 页 / 388 K
品牌: TI [ TEXAS INSTRUMENTS ]
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WIDEBAND, HIGH-SENSITIVITY  
TRANSIMPEDANCE DESIGN  
The example of Figure 3 gives approximately 23MHz flat  
bandwidth using the 0.8pF feedback compensation. If the  
total output noise is bandlimited to a frequency less than the  
feedback pole frequency, a simple expression for the equiva-  
lent input noise current is given as Equation 3.  
The high GBP and low input voltage and current noise for the  
OPA846 make it an ideal wideband transimpedance ampli-  
fier. Very high transimpedance gains (> 100k) benefit from  
the low input noise current of a JFET-input op amp, such as  
the OPA657. Unity-gain stability in the op amp is not required  
for application as a transimpedance amplifier. One transim-  
pedance design example is shown on the front page of this  
data sheet. Designs that require high bandwidths from a  
large area (high capacitance) detector with relatively low  
transimpedance gain will benefit from the low input voltage  
noise offered by the OPA846. This input voltage noise is  
peaked up over frequency at the output by the diode source  
capacitance, and can, in many cases, become the limiting  
factor to input sensitivity. The key elements of the design are  
the expected diode capacitance (CD) with the reverse bias  
voltage (VB) applied, the desired transimpedance gain (RF),  
and the GBP of the OPA846 (1750MHz). Figure 3 shows a  
design using a 50pF detector diode capacitance and a 10kΩ  
transimpedance gain. With these three variables set (includ-  
ing the parasitic input capacitance for the OPA846 added to  
CD) the feedback capacitor (CF) value can be set to control  
the frequency response. To achieve a maximally flat 2nd-  
order Butterworth frequency response, set the feedback pole  
as shown in Equation 1.  
2
2
E 2πFC  
4kT  
RF  
EN  
RF  
(
)
N
D
IEQ = IN2  
+
+
+
(3)  
3
Where:  
IEQ = equivalent input noise current if the output noise is  
bandlimited to F < 1/(2πRFCF)  
IN = input current noise for the op amp inverting input  
EN = input voltage noise for the op amp  
CD = diode capacitance  
F = bandlimiting frequency in Hz (usually a post filter prior  
to further signal processing)  
4kT = 1.6E 20J at T = 290K  
Evaluating this expression up to the feedback pole frequency  
at 16.1MHz for the circuit of Figure 3 gives an equivalent  
input noise current of 4.9pA/Hz. This is much higher than  
the 2.8pA/Hz for just the op amp. This result is dominated  
by the last term in the equivalent input noise current calcu-  
lation from Equation 3. It is essential in this case to use a low-  
voltage noise op amp. For example, if a slightly higher input  
noise voltage, but otherwise identical op amp, was used  
instead of the OPA846 amplifier in this application noise  
amplifier (say 2.0nV/Hz), the total input-referred current  
1
GBP  
=
(1)  
2πRFCF  
4πRFCD  
noise would increase to 7.0pA/Hz  
.
The output DC error for the circuit of Figure 3 is minimized by  
including the 10kto ground on the noninverting input. This  
reduces the impact at the output of input bias current errors  
to the offset current times the feedback resistor. To minimize  
the output noise contribution of this resistor, a 0.01µF capaci-  
tor is included in parallel. Worst-case output DC error for the  
circuit of Figure 3 at 25°C is:  
+5V  
Power-supply  
decoupling not shown.  
10kΩ  
0.01µF  
VO = ID RF  
OPA846  
5V  
RF  
10kΩ  
VOS = ±0.6mV (input offset voltage) ± 0.35µA (input offset  
current) 10k= ±4.1mV  
λ
CD  
50pF  
Worst-case output offset DC drift is over the 0°C to 70°C span  
is dVOS/dT = ±1.5µV/°C (input offset drift) ± 2nA/C (input  
offset current drift) 10k= ±21.5µV/°C  
CF  
0.8pF  
ID  
VB  
Improved output DC precision and drift is possible, particu-  
larly at higher transimpedance gains, using the JFET input of  
the OPA657. The JFET input removes the input bias current  
from the error equation (eliminating the need for the resistor  
to ground on the noninverting input), leaving only the input  
offset voltage and drift as an output error term.  
FIGURE 3. Wideband, Low Noise, Transimpedance Amplifier.  
Adding the common-mode and differential-mode input capaci-  
tance (1.8 + 2.0)pF to the 50pF diode source capacitance of  
Figure 3, with a 10ktransimpedance gain using the 1750MHz  
GBP for the OPA846, requires a feedback pole set to 16.1MHz.  
This requires a 1pF total feedback capacitance. Typical sur-  
face-mount resistors have 0.2pF parasitic capacitance leaving  
a required extrinsic 0.8pF value, as shown in Figure 3.  
Equation 2 gives the approximate 3dB bandwidth, if CF is set  
using Equation 1.  
Included in the characteristic curves are transimpedance  
frequency response curves for a fixed 10kgain over vari-  
ous detector diode capacitance settings. These curves, along  
with the test circuit, are repeated in Figure 4. As the photo-  
GBP  
f3dB  
=
Hz  
(
)
(2)  
2πRFCD  
OPA846  
SBOS250C  
11  
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