OPA835_15 Datasheet, PDF (28/57 Pages) Texas Instruments – OPAx835 Ultralow-Power, Rail-to-Rail Out, Negative Rail In, VFB Op Amp

OPA835, OPA2835
SLOS713H – JANUARY 2011 – REVISED NOVEMBER 2015
Device Functional Modes (continued)
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Signal and bias
from previous stage
V_SIG
2.5 V
5V
R₁
5V
2.5 V
REF
R_G
C
OPA 835
R_F
R_O
V_OUT
GV_SIG
2.5 V
Signal and bias to
next stage
Figure 60. Noninverting Single Supply With Reference
Figure 61 shows a similar noninverting single supply scenario with the reference generator replaced by the
Thevenin equivalent using resistors and the positive supply. R_G’ and R_G” form a resistor divider from the 5-V
supply and are used to bias the negative side with their parallel sum equal to the equivalent R_G to set the gain.
To cancel the voltage offset that would otherwise be caused by the input bias currents, R₁ is chosen to be equal
to R_F in parallel with R_G’ in parallel with R_G” (R₁= R_F || R_G’ || R_G”). For example if gain of 2 is required and R_F = 2
kΩ, selecting R_G’ = R_G” = 4 kΩ gives equivalent parallel sum of 2 kΩ, sets the gain to 2, and references the input
to mid supply (2.5 V). R₁ is then set to 1 kΩ for bias current cancellation, which can be lower cost, but consider
the extra current draw required in the resistor divider.
Signal and bias
from previous stage
V_SIG
2.5 V
R₁
R_G’
5V
OPA 835
5 V _RG”
R_F
R_O
V_OUT
GV_SIG
2.5 V
Signal and bias to
next stage
Figure 61. Noninverting Single Supply With Resistors
Figure 62 shows a typical inverting amplifier situation. With 5-V single supply, a mid-supply reference generator
is needed to bias the positive side through R₁. To cancel the voltage offset that would otherwise be caused by
the input bias currents, R₁ is chosen to be equal to R_F in parallel with R_G. For example, if a gain of –2 is required
and R_F = 2 kΩ, select R_G = 1 kΩ to set the gain and R₁ = 667 Ω for bias current cancellation. The value for C is
dependent on the reference, but at least 0.1 µF is recommended to limit noise into the op amp.
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