LINEAR TECHNOLOGY LTC6081 Technical data

DESIGN IDEAS L

V

RRR

R

V V

OUT IN IN

= +





















−

( )

1

2 105

3

2 1

V V V

V

R

R

V V

OS OSB OSA

OSC

OSB OSA

= − +

+

≈ −

1

2 1

0

+

–

V

OUT

V

IN1

–

+

LTC6081/2

A

LTC6081/2

C

R4R3

V

IN2

+

–

LTC6081/2

B

R6R5

+

–

V

OUT

V

IN1

–

+

LTC6081/2

A

LTC6081/2

C

R4R3

R1

V

IN2

+

–

LTC6081/2

B

R6R5

R2

R0

V

OSDRIFT

(µV/°C)

NUMBER OF AMPLIFIERS (OUT OF 100)

30

25

20

15

10

5

0

LTC6081MS8
TA = –40°C TO 125°C
VS = 3V
VCM = 0.5V

0.20 0.30–0.10–0.20 0 0.10

CMOS Op Amp Outperforms Bipolar
Amps in Precision Applications

Introduction

The LTC6081 and LTC6082 are dual
and quad low offset, low drift, low noise
CMOS operational amplifiers with rail­to-rail input and output stages. Their

0.8µV/°C maximum offset drift, 1pA
input bias current, 1.3µV
10Hz noise, 120dB open loop gain and
110dB CMRR and PSRR make them
perfect for precision applications. The
LTC6081 and LTC6082 have a gain
bandwidth product of 3.6MHz, with
each amplifier only consuming about
330µA current for a supply voltage of

2.7 to 5.5V. The 10-lead DFN pack­age of the LTC6081 offers a shutdown
function to reduce each amplifier’s
supply current to 2µA.

Superior Precision
CMOS Op Amp

Bipolar amplifiers can have low offset
and low offset drift, but their nA level
input bias current make them inap­propriate for high input impedance
applications such as photodiode
amplifiers. CMOS amplifiers usually
offer inferior offset drift, CMRR, and
PSRR specifications and therefore
are not suitable for precision applica­tions. Chopper stabilized amplifiers,
also known as zero drift amplifiers,
can achieve superior offset and offset
drift by means of offset cancellation,
but have clock noise and fold-back
noise due to sampling. LTC6081 and
LTC6082, however, are continuous
time CMOS operational amplifiers,
which use a patented methodology
to improve their offset voltage, offset
voltage drift and CMRR. They combine
the features of low input bias current,
low offset drift and low noise.

Instrumentation Amplifier

Figure 2 shows a typical three op amp
instrumentation amplifier. If R1 = R2,
R3 = R5 and R4 = R6, then

Linear Technology Magazine • March 2008

by Hengsheng Liu

R4/R3 and R6/R5 is critical for CMRR.
Gain can be changed by simply chang­ing R0 without affecting the resistor
matching.

The input referred offset of the

of 0.1Hz to

p-p

Figure 1. VOS drift histogram of LTC6081

In this two stage structure, the dif­ferential voltage passes through the
first gain stage with gain of 1 + 2R1/R0
while the common mode voltage has
only unity gain at the first stage, thus
improving CMRR. Ratio matching of

Figure 2. Typical three op amp structure of instrumentation amplifier

Figure 3. Instrumentation amplifier with unity gain buffers

amplifier is

Statistically, the total VOS is √2
times the VOS of a single op amp.
Since a single LTC6081 op amp drifts
less than 0.8µV/°C, the amplifier in
Figure 2 will drift less than 1.1µV/°C.
One drawback of the circuit in Figure
2 is its common mode operating range
is no longer rail-to-rail. Assuming

3535

L DESIGN IDEAS

R
R

R
R

6
5

1

4
3

= +

( )

ε

20log

A

V

ε

V V

R

R

V V

IN CM IN DM

–

( ) ( )

< ± <

+

2

1

0

V

R

R

V V

V

R

R

V

IN DM IN CM

IN DM

–

( ) ( )

( )

+ < <

−

+

2

2

1

0

1

0

+

–

V

OUT

V

IN1

LTC6081/2

R4R3

V

IN2

R6R5

5V

5V

–

+

–

+

1M

1M

10k

0.1µF

1µF

2.49M

V

OUT

= 10mV/°C

0°C TO 500°C

100pF

K

LT1025

R

–

SENSOR: OMEGA 5TC-TT-K-30-36 K-TYPE THERMOCOUPLE
1M RESISTORS PROTECT CIRCUIT TO ±350V WITH NO PHASE REVERSAL OF AMPLIFIER OUTPUT
1pA MAX I

BIAS

TRANSLATES TO 0.05°C ERROR

90µV VOS → 2°C OFFSET

1/2

LTC6081

the differential and common mode
input voltage are V

IN(DM)

and V

IN(CM)

respectively, the output voltages of op
amp A and B are then V
R0)V

IN(DM)

and V

+ (2R1/R0)V

IN(CM)

IN(CM)

– (2R1/

IN(DM)

respectively. So

where V+ and V– are the positive and
negative supply voltage respectively.
The larger the first stage gain or input
differential signal is, the narrower
the input common mode range is. To
widen the input common mode range,
the first stage gain can be reduced,
but this will compromise CMRR per­formance.

Figure 3 is a reduced circuit of Fig­ure 2 with a unity gain buffer at the
front stage. This circuit can achieve
rail-to-rail input range. As mentioned

previously, it won’t have the high
CMRR of the circuit in Figure 2 since
we reduced the front stage gain to
unity. If the input resistance require­ment can be eased, Figure 3 can be
reduced to Figure 4, a single stage
difference amplifier. The impedance of
the non-inverting and inverting inputs
are R3 and R5 + R6, respectively. An
obvious advantage of the LTC6081 is
its super low input bias current. Even
with a 1MΩ input resistor R3 or R5,
the less than 1pA input bias current
of LTC6081 will add less than 1µV
to VOS.

The above discussion assumes
a perfect matching of R4/R3 and
R6/R5. If

then the CMRR degrades to

where AV is the differential gain of
the instrumentation amplifier. For
example, at gain of 10, to achieve 80dB

Figure 4. Difference amplifier
with no input buffers

CMRR, mismatch of R4/R3 and R6/R5
should be less than 0.1%. This is true
for all the above three circuits. The
advantage of the circuit in Figure 2 is
that gain can be put at the front stage
to ease the matching requirements of
the second stage. Matching of R1 and
R2 in Figure 2 is not important.

Thermocouple Amplifier

Figure 5 shows the LTC6081 in a
thermocouple amplifier. The 1MΩ re­sistors protect the circuit up to ±350V
with no phase reversal to amplifier
output. The 1pA maximum IBIAS of
the LTC6081 translates to a miniscule

0.05°C temperature error with the
1MΩ input protection resistor. The
±90µV offset over the entire operating
temperature range ensures a less than
2°C temperature offset.

LTM4605/07, continued from page 19

Conclusion

The LTM4605 and LTM4607 µModule
regulators simplify the design of buck­boost power supplies. Their low profile
15mm × 15mm × 2.8mm packages
and minimal component count help
free up valuable PCB area. High input
and high output ratings suit these

36

36

Figure 5. Thermocouple amplifier

regulators to networking, industrial,
automotive systems and high power
battery-operated devices. Their opti­mized internal 4-switch architecture
provides high efficiency and high
performance. Overall, the LTM4605
and LTM4607 reduce product design
and test time with a mix of high per-

Conclusion

The LTC6081 and LTC6082 are high
performance dual and quad op amps
combining excellent noise, offset drift,
CMRR, PSRR and input bias current
specifications. They perform in a vari­ety of topologies without compromising
performance. LTC6081 is available in
8-lead MSOP and 10-lead DFN pack­ages. LTC6082 is available in 16-lead
SSOP and DFN packages.

formance features, flexible settings
and ease-of-use.

Notes

1 For more about layout with Kelvin sense resis-

tors, see “Using Current Sensing Resistors with
Hot Swap Controllers and Current Mode Voltage
Regulators” by Eric Trelewicz in Linear Technology

Magazine, September 2003, page 34

Linear Technology Magazine • March 2008

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L