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參數(shù)資料
| 型號： | AB-015 |
| 元件分類： | 差分放大器 |
| 英文描述： | AB-015 - EXTENDING THE COMMON-MODE RANGE OF DIFFERENCE AMPLIFIERS |
| 中文描述： | 抗體- 015 -擴(kuò)展共模差分放大器范圍 |
| 文件頁數(shù)： | 4/5頁 |
| 文件大?。?/td> | 57K |
| 代理商： | AB-015 |
4
In the circuit of Figure 5, the true common-mode signal
drives the INA117 reference, V
. Often it is adequate to
drive the reference in response to the common-mode signal
at just one input pin (either pin 2 or 3). In this case one
common-mode sense resistor (either R
or R
) can be omit-
ted. The value of the feedback resistor, R
6
, must then be
doubled.
The same principles can be applied to the INA105 difference
amplifier as shown in Figure 6. Using the same 3V, 2V rules,
the following relationships apply for the INA105.
V
CM
=
±
{2 ((V+) – 3V) – V
X
}
V
X
= (V–) + 2V
R
6
= –0.5M
V
X
/V
CM
OUTPUT RANGE LIMITATIONS
Keep in mind that with any of these techniques, the
common-mode range refers to the input of the difference
amp only. To make use of the extended common-mode
range, the output swing limitations of the difference amp
must also be observed.
The output of the INA117 or INA105 is guaranteed to swing
at least
±
10V on
±
15V power supplies. However, a negative
output can actually swing to within 3V of the negative power
supply (to –12V on
±
15V supplies).
With zero differential input voltage to the difference amplifier,
the output will be at zero volts with respect to the reference
connection, V
. The circuits in Figure 1 or 3 will not work
with zero differential input. Since the difference amp reference
pin is connected to V–, the output of the difference amp
would saturate to its negative swing limit in an attempt to
swing to V–. For the circuit to work, the differential input
must be at least 3V so that the output of the difference
amplifier is at 3V from V–. The input to the difference
amplifier can be either +3V or –3V and the input connec-
tions (pins 2 and 3) can be interchanged to provide the
proper output swing.
The circuits in Figures 2 and 4 will function with zero
differential input since the reference is connected 3.3V from
V–. Output swing in response to input signal must, however,
always be in the positive direction. As before, the inputs of
both difference amplifiers can be connected to accommodate
any polarity of input/output swing.
If bipolar output swing is required, offset from the rail must
be large enough to accommodate the common-mode offset
as well as the output swing. When using the Figure 5 or 6
circuit, the V
X
terms in the equations must be replaced by:
[(V–) + 3V + V
SW
] for negative swings
and by
[(V+) – 5V – V
SW
] for positive swings
Where:
V
SW
= difference amp output swing relative to reference,
V
X
.
(V
SW
= difference amp differential input since the gain = 1)
The boosted common-mode range for positive inputs is
different than for negative inputs due to the differences in
the difference amplifier output swing limitations:
For positive common-mode inputs:
V
CMH
= 20 ((V+) – 3V) – 19 V
XH
R
6H
= –0.5M
V
XH
/V
CMH
V
XH
= [(V–) + 3V + V
SW
]
For negative common-mode inputs:
V
CML
= 20 ((V–) + 3V) – 19 V
XL
R
6L
= –0.5M
V
XL
/V
CMH
V
XL
= [(V+) – 5V – V
SW
]
Where:
V
CMH
= highest common-mode input voltage [V]
V
CML
= lowest common-mode input voltage [V]
Since only one value for R6 can be used, the smaller value
must be selected if the common-mode input is bipolar. The
total common-mode swing is limited by this value.
Reducing the difference amplifier output swing increases the
possible common-mode input range. If a higher output
swing is needed, add gain after the difference amplifier as
shown in Figure 7. In this circuit an optional f
= 1kHz
filter is added ahead of the gain stage. The filter reduces
output noise at the expense of signal bandwidth reduction. If
filtering is not desired, connect the difference amp output
directly to the noninverting input of the output amplifier.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
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