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參數(shù)資料
| 型號： | OPA685 |
| 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(biāo)使用禁用 |
| 文件頁數(shù)： | 26/28頁 |
| 文件大?。?/td> | 283K |
| 代理商： | OPA685 |
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26
OPA685
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA685 provides ex-
ceptional bandwidth in high gains, giving fast pulse settling
but only moderate DC accuracy. The typical specifications
show an input offset voltage comparable to high-speed
voltage-feedback amplifiers, however, the two input bias
currents are somewhat higher and are unmatched. Although
bias current cancellation techniques are very effective with
most voltage-feedback op amps, they do not generally re-
duce the output DC offset for wideband current-feedback op
amps. Since the two input bias currents are unrelated in both
magnitude and polarity, matching the source impedance
looking out of each input to reduce their error contribution
to the output is ineffective. Evaluating the configuration of
Figure 1, using worst-case +25
°
C input offset voltage and
the two input bias currents, gives a worst-case output offset
range equal to:
±
(NG
V
OS
) + (I
BN
R
S
/2
NG)
±
(I
BI
R
F
)
where NG = non-inverting signal gain
=
±
(8
3.5mV) + (90
μ
A
25
8)
±
(402
100
μ
A)
=
±
28mV + 18mV
±
40mV
= –50mV
→
+86mV
A fine-scale output offset null, or DC operating point adjust-
ment, is often required. Numerous techniques are available
for introducing DC offset control into an op amp circuit.
Most simple adjustment techniques do not correct for tem-
perature drift. It is possible to combine a lower speed,
precision op amp with the OPA685 to get the DC accuracy
of the precision op amp along with the signal bandwidth of
the OPA685. Figure 17 shows a non-inverting G = +10
circuit that holds an output offset voltage less than
±
1.0mV
over-temperature with > 300MHz bandwidth.
This DC-coupled circuit provides very high signal band-
width using the OPA685. At lower frequencies, the output
voltage is attenuated by the signal gain and is compared to
the original input voltage at the inputs of the OPA227 (a low
cost, precision voltage-feedback op amp with 8MHz gain
bandwidth product). If these two don’t agree at low frequen-
cies, the OPA227 sums in a correcting current through the
2.55k
inverting summing path. Several design consider-
ations will allow this circuit to be optimized. First, the
feedback to the OPA227’s non-inverting input must be
precisely matched to the high-speed signal gain. Making the
249
resistor to ground an adjustable resistor would allow
the low and high frequency gains to be precisely matched.
Secondly, the crossover frequency region where the OPA227
passes control to the OPA685 must occur with exceptional
phase linearity. These two issues reduce to designing for
pole/zero cancellation in the overall transfer function. Using
the 2.55k
resistor will nominally satisfy this requirement
for the circuit of Figure 17. Perfect cancellation over process
and temperature is not possible. However, this initial resistor
setting and precise gain matching will minimize long-term
pulse settling perturbations.
DISABLE OPERATION
The OPA685 provides an optional disable feature that may
be used either to reduce system power or to implement a
simple channel multiplexing operation. If the DIS control
pin is left unconnected, the OPA685 will operate normally.
To disable, the control pin must be asserted low. Figure 18
shows a simplified internal circuit for the disable control
feature.
FIGURE 17. Wideband, Precision, G = +10 Composite
Amplifier.
25k
110k
15k
I
Control
–V
S
+V
S
V
DIS
Q1
FIGURE 18. Simplified Disable Control Circuit.
In normal operation, base current to Q1 is provided through
the 110k
resistor while the emitter current through the
15k
resistor sets up a voltage drop that is inadequate to
turn on the two diodes in Q1’s emitter. As V
DIS
is pulled
LOW, additional current is pulled through the 15k
resistor,
eventually turning on these two diodes (
≈
100
μ
A). At this
OPA685
365
2.55k
20
41.2
DIS
+5V
–5V
V
O
Power supply
decoupling not shown
OPA227
–5V
+5V
V
I
2.26k
249
680pF
680pF
226
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA685N | 制造商:BB 制造商全稱:BB 功能描述:Ultra-Wideband, Current-Feedback OPERATIONAL AMPLIFIER With Disable TM |
| 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(fēng)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 |
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