OPA2835 Datasheet, PDF (25/53 Pages) National Semiconductor (TI) – Ultra Low-Power, Rail-to-Rail Out, Negative Rail In, VFB Op Amp

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OPA835
OPA2835
SLOS713E – JANUARY 2011 – REVISED JULY 2013
VSIG
VREF
V_IN
49.9
OPA 835
2k
+
VREF
2k
49.9
OPA 835
1k
Figure 56. CAT 5 Line Driver with Gain = 2 V/V (6 dB)
VSIG
VOUT+ VREF
-VSIG
VOUT- VREF
Differential to Signal Ended Amplifier
Figure 57 shows a differential amplifier that is used to convert differential signals to single-ended and provides
gain (or attenuation) and level shifting. This circuit can be used in applications like a line receiver for converting a
differential signal from a CAT 5 cable to single-ended.
If we set V_IN+ = V_CM + V_SIG+ and V_IN- = V_CM + V_SIG-, then
( ) V_OUT =
V_IN+
-
V_IN-
´
æ
ç
R_F
è ^RG
ö
÷
ø
+ V_REF
(4)
G = ^RF
The signal gain of the circuit is set by:
R_{G , VCM is rejected, and VREF provides a level shift around which}
the output signal swings. The single ended output signal is in-phase with the differential input signal.
V_SIG-
V_CM
R_G
V_IN-
V_IN+
R_G
V_CM
V_SIG+
R_F
R_F
OPA 835
V_OUT
G[(V_SIG+)-(V_SIG-)]
V_REF
V_REF
Figure 57. Differential to Single Ended Amplifier
Line termination can be accomplished with a resistor shunt across the input. The impedance seen differential
from the line will be the resistor value in parallel with the amplifier circuit. For low gain and low line impedance
the resistor value to add is approximately the impedance of the line. For example, if 100 Ω CAT5 cable is used
with a gain of 1 amplifier and R_F = R_G = 2 kΩ, adding a 100 Ω shunt across the input will give a differential
impedance of 99 Ω; this should be adequate for most applications.
For best CMRR performance, resistors must be matched. Assuming CMRR ≈ the resistor tolerance; so 0.1%
tolerance will provide about 60 dB CMRR.
Differential to Differential Amplifier
Figure 58 shows a differential amplifier that is used to amplify differential signals. This circuit has high input
impedance and is often used in differential line driver applications where the signal source is a high impedance
driver like a differential DAC that needs to drive a line.
If we set V_IN± = V_CM + V_SIG± then
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