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LTC6261/LTC6262/LTC6263

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For more information www.linear.com/LTC6261

operAtion

ApplicAtions inForMAtion

The LTC6261 family input signal range extends slightly

beyond the negative and positive power supplies. The

output can even extend all the way to the negative supply

with the proper external pull-down current source. Figure

1 depicts a Simplified Schematic of the amplifier. The

input stage is comprised of two differential amplifiers, a

PNP stage Q1/Q2 and NPN stage Q3/Q4 that are active

over different ranges of common mode input voltage. The

PNP stage is active between the negative power supply

to approximately 1V below the positive supply. As the

input voltage approaches the positive supply, transistor

Q5 will steer the tail current I1 to the current mirror Q6/

Q7, activating the NPN differential pair and the PNP pair

becomes inactive for the remaining input common mode

range. Also for the input stage, devices Q17, Q18 and

Q19 act to cancel the bias current of the PNP input pair.

When Q1/Q2 is active, the current in Q16 is controlled to

be the same as the current Q1/Q2. Thus, the base current

of Q16 is normally equal to the base current of the input

devices of Q1/Q2. Similar circuitry (not shown) is used to

cancel the base current of Q3/Q4. The buffer and output

bias stage uses a special compensation technique to take

full advantage of the process technology to drive high

capacitive loads. The common emitter topology of Q14/

Q15 enables the output to swing from rail-to-rail.

Low Supply Voltage and Low Power Consumption

The LTC6261 family of operational amplifiers can oper-

ate with power supply voltages from 1.8V to 5.25V. Each

amplifier draws 240µA. The low supply voltage capability

and low supply current are ideal for portable applications.

High Capacitive Load Driving Capability and Wide

Bandwidth

The LTC6261 family is optimized for wide bandwidth and

low power applications. They have an extremely high

gain-bandwidth to power ratio and are unity gain stable

(seeTypicalPerformanceCharacteristics,CapacitiveLoad

Handling). Higher gain configurations tend to have better

capacitive drive capability than lower gain configurations

due to lower closed loop bandwidth and hence higher

phase margin.

Low Input Referred Noise

The LTC6261 family provides a low input referred noise of

13nV/√Hzat10kHz.Theaveragenoisevoltagedensityover

1MHz of bandwidth is less than 15nV/√Hz. The LTC6261

family is ideal for low noise and low power signal process-

ing applications.

Low Input Offset Voltage

The LTC6261 family has a low offset voltage of 1mV maxi-

mum. The offset voltage is trimmed with a proprietary

algorithm to ensure low offset voltage over the entire

common mode voltage range.

Low Input Bias Current

TheLTC6261familyusesabiascurrentcancellationcircuit

tocompensateforthebasecurrentoftheinputtransistors.

When the input common mode voltage is within 200mV of

either rail, the bias cancellation circuit is no longer active.

For common mode voltages ranging from 0.2V above the

negative supply to 0.2V below the positive supply, the

low input bias current allows the amplifiers to be used in

applications with high resistance sources.

Ground Sensing and Rail-to-Rail Output

The LTC6261 family has excellent output drive capability,

delivering over 10mA of output drive current. The output

stageisarail-to-railtopologythatiscapableofswingingto

within 250mV of either rail. If output swing to the negative

rail is required, an external pull down resistor to a negative

supply can be added. For 5V/0V op amp supplies, a pull

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