OPA2835 Datasheet, PDF (30/53 Pages) National Semiconductor (TI) – Ultra Low-Power, Rail-to-Rail Out, Negative Rail In, VFB Op Amp

OPA835
OPA2835
SLOS713E – JANUARY 2011 – REVISED JULY 2013
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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 63. Non-Inverting Single Supply with Reference
Figure 64 shows a similar non-inverting 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 5V
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 in 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 1kΩ for bias current cancellation. This can be lower cost, but note
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 64. Non-Inverting Single Supply with Resistors
Figure 65 shows a typical inverting amplifier situation. With 5V single supply, a mid supply reference generator is
needed to bias the positive side via 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 gain of -2 is required and R_F = 2
kΩ, select R_G = 1 kΩ to set the gain and R₁ = 665 Ω for bias current cancellation. The value for C is dependent
on the reference, but at least 0.1µF is recommended to limit noise into the op amp.
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