參數(shù)資料
型號: THS3121CDR
廠商: TEXAS INSTRUMENTS INC
元件分類: 音頻/視頻放大
英文描述: 1 CHANNEL, VIDEO AMPLIFIER, PDSO8
封裝: GREEN, PLASTIC, MS-012AA, SOIC-8
文件頁數(shù): 11/35頁
文件大?。?/td> 955K
代理商: THS3121CDR
0
200
400
600
800
1000
1200
1400
100 k
1 M
10 M
100 M
1 G
f Frequency Hz
Powerdown
Output
Impedance
Gain = 2
RF = 649
VS = ±15 V and ±5 V
www.ti.com........................................................................................................................................ SLOS420E – SEPTEMBER 2003 – REVISED OCTOBER 2009
Figure 58 shows the total system output impedance
POWER-DOWN REFERENCE PIN
which includes the amplifier output impedance in
OPERATION
parallel with the feedback plus gain resistors, which
In addition to the power-down pin, the THS3120
cumulate to 1298
. Figure 47 shows this circuit
features a reference pin (REF) which allows the user
configuration for reference.
to control the enable or disable power-down voltage
levels applied to the PD pin. In most split-supply
applications, the reference pin is connected to
ground. In either case, the user needs to be aware of
voltage-level thresholds that apply to the power-down
pin. Table 2 shows examples and illustrate the
relationship between the reference voltage and the
power-down thresholds. In the table, the threshold
levels are derived by the following equations:
PD
≤ REF + 0.8 V for enable
PD
≥ REF + 2 V for disable
where the usable range at the REF pin is
VS– ≤ VREF ≤ (VS+ – 4 V).
The recommended mode of operation is to tie the
REF pin to midrail, thus settings the enable/disable
threshold to V(midrail) + 0.8 V and V(midrail) = 2 V
Figure 58. Power-down Output Impedance vs
respectively.
Frequency
Table 2. Power-Down Threshold Voltage Levels
As with most current-feedback amplifiers, the internal
REFERENCE
architecture places some limitations on the system
SUPPLY
PIN
ENABLE
DISABLE
when in power-down mode. Most notably is the fact
VOLTAGE (V)
LEVEL (V)
that the amplifier actually turns ON if there is a ±0.7 V
±15, ±5
0
0.8
2
or greater difference between the two input nodes
±15
2
2.8
4
(V+ and V–) of the amplifier. If this difference
±15
–2
–1.2
0
exceeds ±0.7 V, the output of the amplifier creates an
±5
1
1.8
3
output voltage equal to approximately [(V+) – (V–) –
0.7 V] × Gain. Also, if a voltage is applied to the
±5
–1
–0.2
1
output while in power-down mode, the V– node
30
15
15.8
17
voltage is equal to VO(applied) × RG/(RF + RG). For low
10
5
5.8
7
gain configurations and a large applied voltage at the
output, the amplifier may actually turn ON due to the
Note that if the REF pin is left unterminated, it floats
aforementioned behavior.
to
the
positive
rail
and
falls
outside
of
the
recommended operating range given above (VS–
The time delays associated with turning the device on
VREF ≤ VS+ – 4 V). As a result, it no longer serves as
and off are specified as the time it takes for the
a reliable reference for the PD pin, and the
amplifier to reach either 10% or 90% of the final
enable/disable thresholds given above no longer
output voltage. The time delays are in the order of
apply. If the PD pin is also left unterminated, it floats
microseconds because the amplifier moves in and out
to the positive rail and the device is disabled. If
of the linear mode of operation in these transitions.
balanced, split supplies are used (±VS) and the REF
and PD pins are grounded, the device is enabled.
Copyright 2003–2009, Texas Instruments Incorporated
19
Product Folder Link(s): THS3120 THS3121
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