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參數(shù)資料
| 型號： | OPA685N |
| 英文描述： | Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM |
| 中文描述： | 超寬帶，電流反饋運算放大器商標使用禁用 |
| 文件頁數(shù)： | 23/28頁 |
| 文件大?。?/td> | 283K |
| 代理商： | OPA685N |
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23
OPA685
R
I
, the buffer output impedance, is a critical portion of the
bandwidth control equation. For the OPA685, it is typically
about 19
for
±
5V operation and 23
for single +5V operation.
A current-feedback op amp senses an error current in the
inverting node (as opposed to a differential input error
voltage for a voltage-feedback op amp) and passes this on to
the output through an internal frequency dependent
transimpedance gain. The Typical Performance Curves show
this open-loop transimpedance response. This is analogous
to the open-loop voltage gain curve for a voltage-feedback
op amp. Developing the transfer function for the circuit of
Figure 14 gives Equation 6:
(6)
This is written in a loop gain analysis format where the
errors arising from a non-infinite open-loop gain are shown
in the denominator. If Z(s) were infinite over all frequen-
cies, the denominator of Equation 6 would reduce to 1 and
the ideal desired signal gain shown in the numerator would
be achieved. The fraction in the denominator of Equation 6
determines the frequency response. Equation 7 shows this as
the loop gain equation:
(7)
If 20 log(R
F
+ NG R
I
) were superimposed of the open-
loop transimpedance plot, the difference between the two
would be the loop gain at a given frequency. Eventually,
Z(s) rolls off to equal the denominator of Equation 7, at
which point the loop gain has reduced to 1 (and the curves
have intersected). This point of equality is where the
amplifier’s closed-loop frequency response given by Equa-
tion 6 will start to roll off, and is exactly analogous to the
frequency at which the noise gain equals the open-loop
voltage gain for a voltage-feedback op amp. The difference
here is that the total impedance in the denominator of
Equation 7 may be controlled separately from the desired
signal gain (or NG).
The OPA685 is internally compensated to give a maximally
flat frequency response for R
F
= 402
at NG = 8 on
±
5V
supplies. Evaluating the denominator of Equation 7 (which
is the feedback transimpedance) gives an optimal target of
554
. As the signal gain changes, the contribution of the
NG R
I
term in the feedback transimpedance will change,
but the total can be held constant by adjusting R
F
. Equation
8 gives an approximate equation for optimum R
F
over signal
gain:
(8)
V
O
V
I
=
α
1
+
R
F
R
F
+
R
I
1
+
R
F
R
G
Z
(S)
1
+
R
G
=
α
NG
1
+
R
F
+
R
I
NG
Z
(S)
NG
=
1
+
R
F
R
G
Z
(S)
R
F
+
R
I
NG
=
Loop Gain
As the desired signal gain increases, this equation will
eventually predict a negative R
F
. A somewhat subjective
limit to this adjustment can also be set by holding R
G
to a
minimum value of 10
. Lower values will load both the
buffer stage at the input and the output stage if R
F
gets too
low, actually decreasing the bandwidth. Figure 15 shows the
recommended R
F
versus NG for both
±
5V and a single +5V
operation. The optimum target feedback impedance for +5V
operation used in Equation 8 is 532
,
while the typical
buffer output impedance is 23
. The values for R
F
versus
gain shown here are approximately equal to the values used
to generate the Typical Performance Curves. In some cases,
the values used differ slightly from that shown here in that
the values used in the Typical Performance Curves are also
correcting for board parasitics not considered in the simpli-
fied analysis leading to Equation 8. The values shown in
Figure 15 give a good starting point for design where
bandwidth optimization is desired.
FIGURE 15. Recommended Feedback Resistor vs Noise
Gain.
The total impedance presented to the inverting input may be
used to adjust the closed-loop signal bandwidth. Inserting a
series resistor between the inverting input and the summing
junction will increase the feedback impedance (denominator
of Equation 7), decreasing the bandwidth. The internal
buffer output impedance for the OPA685 is slightly influ-
enced by the source impedance looking out of the non-
inverting input terminal. High source resistors will have the
effect of increasing R
I
, decreasing the bandwidth. For those
single-supply applications which develop a midpoint bias at
the non-inverting input through high valued resistors, the
decoupling capacitor is essential for power supply ripple
rejection, non-inverting input noise current shunting, and
minimizing the high frequency value for R
I
in Figure 14.
Inverting feedback optimization is somewhat complicated
by the impedance matching requirement at the input, as
shown in Figure 2. The resistor values shown in Table III
should be used in this case.
R
NG R
F
I
=
554
–
600
500
400
300
200
100
0
0
2
4
6
8
10
12
14
16
18
20
Noise Gain (V/V)
F
)
V
S
= +5V
V
S
=
±
5V
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| OPA685N/250 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
| OPA685N/3K | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
| OPA685U | 功能描述:高速運算放大器 Ultra-Wideband RoHS:否 制造商:Texas Instruments 通道數(shù)量:1 電壓增益 dB:116 dB 輸入補償電壓:0.5 mV 轉(zhuǎn)換速度:55 V/us 工作電源電壓:36 V 電源電流:7.5 mA 最大工作溫度:+ 85 C 安裝風格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| OPA685U/2K5 | 制造商:未知廠家 制造商全稱:未知廠家 功能描述:Current-Feedback Operational Amplifier |
| OPA686 | 制造商:BB 制造商全稱:BB 功能描述:Wideband, Low Noise, Voltage Feedback OPERATIONAL AMPLIFIER |
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