LPV521 datasheet(21/29 Pages) TI

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VOUT

100 M:

1 M:

OXYGEN SENSOR

LPV521

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SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013

Figure 65. Precision Oxygen Sensor

INPUT STAGE

The LPV521 has a rail-to-rail input which provides more flexibility for the system designer. Rail-to-rail input is

achieved by using in parallel, one PMOS differential pair and one NMOS differential pair. When the common

mode input voltage (VCM) is near V+, the NMOS pair is on and the PMOS pair is off. When VCM is near V−, the

NMOS pair is off and the PMOS pair is on. When VCM is between V+ and V−, internal logic decides how much

current each differential pair will get. This special logic ensures stable and low distortion amplifier operation

within the entire common mode voltage range.

Because both input stages have their own offset voltage (VOS) characteristic, the offset voltage of the LPV521

becomes a function of VCM. VOS has a crossover point at 1.0V below V+. Refer to the ’VOS vs. VCM’ curve in the

Typical Performance Characteristics section. Caution should be taken in situations where the input signal

amplitude is comparable to the VOS value and/or the design requires high accuracy. In these situations, it is

necessary for the input signal to avoid the crossover point. In addition, parameters such as PSRR and CMRR

which involve the input offset voltage will also be affected by changes in VCM across the differential pair transition

region.

OUTPUT STAGE

The LPV521 output voltage swings 3 mV from rails at 3.3V supply, which provides the maximum possible

dynamic range at the output. This is particularly important when operating on low supply voltages.

The LPV521 Maximum Output Voltage Swing defines the maximum swing possible under a particular output

load. The LPV521 output swings 50 mV from the rail at 5V supply with an output load of 100 k

Ω.

DRIVING CAPACITIVE LOAD

The LPV521 is internally compensated for stable unity gain operation, with a 6.2 kHz typical gain bandwidth.

However, the unity gain follower is the most sensitive configuration to capacitive load. The combination of a

capacitive load placed at the output of an amplifier along with the amplifier’s output impedance creates a phase

lag, which reduces the phase margin of the amplifier. If the phase margin is significantly reduced, the response

will be under damped which causes peaking in the transfer and, when there is too much peaking, the op amp

might start oscillating.

In order to drive heavy capacitive loads, an isolation resistor, RISO, should be used, as shown in Figure 66. By

using this isolation resistor, the capacitive load is isolated from the amplifier’s output. The larger the value of

RISO, the more stable the amplifier will be. If the value of RISO is sufficiently large, the feedback loop will be

stable, independent of the value of CL. However, larger values of RISO result in reduced output swing and

reduced output current drive.

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