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| 型號： | OPA694IDR |
| 英文描述： | Wideband, Low-Power, Current Feedback Operational Amplifier |
| 中文描述： | 寬帶，低功耗，電流反饋運算放大器 |
| 文件頁數(shù)： | 16/24頁 |
| 文件大?。?/td> | 382K |
| 代理商： | OPA694IDR |
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SBOS319C SEPTEMBER 2004 REVISED NOVEMBER 2004
www.ti.com
16
NOISE PERFORMANCE
Wideband, current-feedback op amps generally have a
higher output noise than comparable voltage-feedback op
amps. The OPA694 offers an excellent balance between
voltage and current noise terms to achieve low output
noise. The inverting current noise (24pA/
√
Hz) is
significantly lower than earlier solutions, while the input
voltage noise (2.1nV/
√
Hz) is lower than most unity-gain
stable, wideband, voltage-feedback op amps. This low
input voltage noise was achieved at the price of higher
noninverting input current noise (22pA/
√
Hz). As long as
the AC source impedance looking out of the noninverting
node is less than 100
, this current noise will not
contribute significantly to the total output noise. The op
amp input voltage noise and the two input current noise
terms combine to give low output noise under a wide
variety of operating conditions. Figure 10 shows the op
amp noise analysis model with all the noise terms
included. In this model, all noise terms are taken to be
noise voltage or current density terms in either nV/
√
Hz or
pA/
√
Hz.
4kT
R
G
R
G
R
F
R
S
OPA694
I
BI
E
O
I
BN
4kT = 1.6
×
10
20
J
at 290K
E
RS
E
NI
√
4kTR
F
√
4kTR
S
Figure 10. Op Amp Noise Analysis Model
The total output spot noise voltage can be computed as the
square root of the sum of all squared output noise voltage
contributors. Equation (4) shows the general form for the
output noise voltage using the terms shown in Figure 10.
E
O
E
NI
2
I
BN
R
S
2
4kTR
S
NG
2
I
BI
R
F
2
4kTR
F
NG
Dividing this expression by the noise gain (NG =
(1 + R
F
/R
G
)) will give the equivalent input-referred spot
noise voltage at the noninverting input, as shown in
Equation 6.
E
N
E
NI
2
I
BN
R
S
2
4kTR
S
I
BI
R
F
NG
2
4kTR
F
NG
Evaluating these two equations for the OPA694 circuit and
component values (see Figure 1) gives a total output spot
noise voltage of 11.2nV/
√
Hz and a total equivalent input
spot noise voltage of 5.6nV/
√
Hz. This total input-referred
spot noise voltage is higher than the 2.1nV/
√
Hz
specification for the op amp voltage noise alone. This
reflects the noise added to the output by the inverting
current noise times the feedback resistor. If the feedback
resistor is reduced in high-gain configurations (as
suggested previously), the total input-referred voltage
noise given by Equation (5) will approach just the
2.1nV/
√
Hz of the op amp itself. For example, going to a
gain of +10 using R
F
= 178
will give a total input-referred
noise of 2.36nV/
√
Hz.
DC ACCURACY AND OFFSET CONTROL
A current-feedback op amp like the OPA694 provides
exceptional bandwidth in high gains, giving fast pulse
settling, but only moderate DC accuracy. The Electrical
Characteristics show an input offset voltage comparable to
high-speed, voltage-feedback amplifiers. However, the
two input bias currents are somewhat higher and are
unmatched.
Whereas
bias
techniques are very effective with most voltage-feedback
op amps, they do not generally reduce 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:
current
cancellation
±
(NG
×
V
OS
)
±
(I
BN
×
R
S
/2
×
NG)
±
(I
BI
×
R
F
)
where NG = noninverting signal gain
=
±
(2
×
3mV)
±
(20
μ
A
×
25
×
2)
±
(402
×
18
μ
A)
=
±
6mV + 1mV
±
7.24mV =
±
14.24mV
A fine-scale, output offset null, or DC operating point
adjustment, is sometimes required. Numerous techniques
are available for introducing DC offset control into an op
amp circuit. Most simple adjustment techniques do not
correct for temperature drift. It is possible to combine a
lower speed, precision op amp with the OPA694 to get the
DC accuracy of the precision op amp along with the signal
bandwidth of the OPA694. Figure 11 shows a noninverting
G = +10 circuit that holds an output offset voltage less than
±
7.5mV over-temperature with > 150MHz signal
bandwidth.
(4)
(5)
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| OPA694IDRG4 | 功能描述:高速運算放大器 WideBand Low-Power Curr Feedback Amp 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 |
| OPA695 | 制造商:BB 制造商全稱:BB 功能描述:Wideband, Low-Power, Current Feedback Operational Amplifier |
| OPA695ID | 功能描述:高速運算放大器 Ultra-Wideband Current Feedback 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 |
| OPA695ID | 制造商:Texas Instruments 功能描述:IC SM OP-AMP WIDEBAND CFB |
| OPA695IDBVR | 功能描述:高速運算放大器 Ultra-Wideband Current Feedback 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 |
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