OPA2835 Datasheet, PDF (28/53 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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Input Common-Mode Voltage Range
When the primary design goal is a linear amplifier, with high CMRR, it is important to not violate the input
common-mode voltage range (V_ICR) of an op amp.
Common-mode input range low and high specifications in the table data use CMRR to set the limit. The limits are
chosen to ensure CMRR will not degrade more than 3dB below its limit if the input voltage is kept within the
specified range. The limits cover all process variations and most parts will be better than specified. The typical
specifications are from 0.2V below the negative rail to 1.1V below the positive rail.
Assuming the op amp is in linear operation the voltage difference between the input pins is very small (ideally
0V) and input common-mode voltage can be analyzed at either input pin and the other input pin is assumed to
be at the same potential. The voltage at V_IN+ is easy to evaluate. In non-inverting configuration, Figure 53, the
input signal, V_IN, must not violate the V_ICR. In inverting configuration, Figure 53, the reference voltage, V_REF,
needs to be within the V_ICR.
The input voltage limits have fixed headroom to the power rails and track the power supply voltages. For with
single 5 V supply, the linear input voltage range is –0.2 V to 3.9 V and with 2.7 V supply it is –0.2 V to 1.6 V. The
delta from each power supply rail is the same in either case; –0.2 V and 1.1 V.
Output Voltage Range
The OPA835 and OPA2835 are rail-to-rail output (RRO) op amps. Rail-to-rail output typically means the output
voltage can swing to within a couple hundred milli-volts of the supply rails. There are different ways to specify
this; one is with the output still in linear operation and another is with the output saturated. Saturated output
voltages are closer to the power supply rails than linear outputs, but the signal is not a linear representation of
the input. Linear output is a better representation of how well a device performs when used as a linear amplifier.
Both saturation and linear operation limits are affected by the current in the output, where higher currents lead to
more loss in the output transistors.
Data in the ELECTRICAL SPECIFICATIONS tables list both linear and saturated output voltage specifications
with 2kΩ load. Figure 11 and Figure 37 show saturated voltage swing limits versus output load resistance and
Figure 12 and Figure 38 show the output saturation voltage versus load current. Given a light load, the output
voltage limits have nearly constant headroom to the power rails and track the power supply voltages. For
example with 2 kΩ load and single 5 V supply the linear output voltage range is 0.15 V to 4.8 V and with 2.7 V
supply it is 0.15 V to 2.5 V. The delta from each power supply rail is the same in either case; 0.15 V and 0.2 V.
With devices like the OPA835 and OPA2835, where the input range is lower than the output range, it is typical
that the input will limit the available signal swing only in non-inverting gain of 1. Signal swing in non-inverting
configurations in gains > +1 and inverting configurations in any gain is generally limited by the output voltage
limits of the op amp.
Split-Supply Operation (±1.25V to ±2.75V)
To facilitate testing with common lab equipment, the OPA835 EVM SLOU314 is built to allow for split-supply
operation. This configuration eases lab testing because the mid-point between the power rails is ground, and
most signal generators, network analyzers, oscilloscopes, spectrum analyzers and other lab equipment reference
their inputs and outputs to ground.
Figure 61 shows a simple non-inverting configuration analogous to Figure 53 with ±2.5V supply and V_REF equal
to ground. The input and output will swing symmetrically around ground. Due to its ease of use, split supply
operation is preferred in systems where signals swing around ground, but it requires generation of two supply
rails.
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