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參數(shù)資料
| 型號(hào)： | OPA688U |
| 元件分類： | 限幅放大器 |
| 英文描述： | Unity Gain Stable, Wideband VOLTAGE LIMITING AMPLIFIER |
| 中文描述： | 單位增益穩(wěn)定，寬帶電壓限幅放大器 |
| 文件頁數(shù)： | 15/16頁 |
| 文件大?。?/td> | 185K |
| 代理商： | OPA688U |
15
OPA688
The limiters’ DC accuracy depends on attention to detail.
The two dominant error sources can be improved as follows:
Power supplies, when used to drive resistive dividers that
set V
H
and V
L
, can contribute large errors (e.g.,
±
5%).
Using a more accurate source, and bypassing pins 5 and 8
with good capacitors, will improve limiter PSRR.
The resistor tolerances in the resistive divider can also
dominate. Use 1% resistors.
Other error sources also contribute, but should have little
impact on the limiters’ DC accuracy:
Reduce offsets caused by the Limiter Input Bias Currents.
Select the resistors in the resistive divider(s) as described
above.
Consider the signal path DC errors as contributing to
uncertainty in the useable output swing.
The Limiter Offset Voltage only slightly degrades limiter
accuracy.
Figure 9 shows how the limiters affect distortion perfor-
mance. Virtually no degradation in linearity is observed for
output voltage swinging right up to the limiter voltages.
makes the OPA688 an ideal choice for a wide range of high
frequency applications.
Many high speed applications, such as driving A/D convert-
ers, require op amps with low output impedance. As shown
in the
Output Impedance vs Frequency
performance curve,
the OPA688 maintains very low closed-loop output imped-
ance over frequency. Closed-loop output impedance in-
creases with frequency since loop gain decreases with fre-
quency.
THERMAL CONSIDERATIONS
The OPA688 will not require heat-sinking under most oper-
ating conditions. Maximum desired junction temperature
will set a maximum allowed internal power dissipation as
described below. In no case should the maximum junction
temperature be allowed to exceed 175
°
C.
The total internal power dissipation (P
D
) is the sum of
quiescent power (P
DQ
) and the additional power dissipated
in the output stage (P
DL
) while delivering load power. P
DQ
is simply the specified no-load supply current times the total
supply voltage across the part. P
DL
depends on the required
output signals and loads. For a grounded resistive load, and
equal bipolar supplies, it is at a maximum when the output
is at 1/2 either supply voltage. In this condition, P
DL
= V
S2
/
(4R
L
) where R
L
includes the feedback network loading.
Note that it is the power in the output stage, and not in the
load, that determines internal power dissipation.
The operating junction temperature is: T
J
= T
A
+ P
D
θ
JA
,
where T
A
is the ambient temperature.
For example, the maximum T
J
for a OPA688U with G = +2,
R
FB
= 402
, R
L
= 100
, and
±
V
S
=
±
5V at the maximum
T
A
= +85
°
C is calculated as:
FIGURE 10. Offset Voltage Trim.
HARMONIC DISTORTION NEAR LIMIT VOLTAGES
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90
± Limit Voltage (V)
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2
V
O
= 0V
±1Vp
f
1
= 5MHz
R
L
= 500
HD2
HD3
FIGURE 9. Harmonic Distortion Near Limit Voltages.
R2
OPA688
R3 = R
1
|| R
2
R1
R
TRIM
47k
+V
S
V
O
–V
S
V
IN
or Ground
0.1μF
NOTES: (1) R
is optional and minimizes output offset
due to input bias currents. (2) Set R
1
<< R
TRIM
.
P
DQ
= 10
V
20
mA
)
= 200
mW
P
DL
=
5
V
(
)
2
4
100
||
804
(
P
D
= 200
mW
+ 70
mW
= 270
mW
T
J
= 85°
C
+ 270
mW
125°
C / W
=119°
C
)
= 70
mW
OFFSET VOLTAGE ADJUSTMENT
The circuit in Figure 10 allows offset adjustment without
degrading offset drift with temperature. Use this circuit with
caution since power supply noise can inadvertently couple
into the op amp.
Remember that additional offset errors can be created by the
amplifier’s input bias currents. Whenever possible, match
the impedance seen by both DC input bias currents using R
3
.
This minimizes the output offset voltage caused by the input
bias currents.
OUTPUT DRIVE
The OPA688 has been optimized to drive 500
loads, such
as A/D converters. It still performs very well driving 100
loads; the specifications are shown for the 500
load. This
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