LTC6241 LINER [Linear Technology], LTC6241 Datasheet - Page 14

• Manufacturer Part Number: LTC6241
• Description: Dual/Quad 18MHz, Low Noise, Rail-to-Rail, CMOS Op Amps
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
LTC6241/LTC6242
Amplifi er Characteristics
Figure 1 is a simplifi ed schematic of the LTC6241, which
has a pair of low noise input transistors M1 and M2. A
simple folded cascode Q1, Q2 and R1, R2 allow the input
stage to swing to the negative rail, while performing level
shift to the Differential Drive Generator. Low offset voltage
is accomplished by laser trimming the input stage.
Capacitor C1 reduces the unity cross frequency and im-
proves the frequency stability without degrading the gain
bandwidth of the amplifi er. Capacitor Cm sets the overall
amplifi er gain bandwidth. The differential drive generator
supplies signals to transistors M3 and M4 that swing the
output from rail-to-rail.
The photo of Figure 2 shows the output response to an
input overdrive with the amplifi er connected as a voltage
follower. If the negative going input signal is less than
a diode drop below V
input signals greater than a diode drop below V
current to 3mA with a series resistor R
inversion.
ESD
The LTC6241 has reverse-biased ESD protection diodes
on all input and outputs as shown in Figure 1. If these
pins are forced beyond either supply, unlimited current
will fl ow through these diodes. If the current is transient
and limited to one hundred milliamps or less, no damage
to the device will occur.
14
DESD1
V
V
DESD3
IN
IN
+
–
V
V
–
–
V
V
+
+
DESD2
DESD4
M1
Figure 1. Simplifi ed Schematic
I
M2
TAIL
U
–
, no phase inversion occurs. For
C1
BIAS
U
V
–
Q1
R1
W
Q2
R2
CM
DIFFERENTIAL
GENERATOR
S
DRIVE
to avoid phase
6241 F01
–
U
, limit the
M3
M4
V
V
V
V
+
–
+
–
DESD5
DESD6
V
O
The amplifi er input bias current is the leakage current of
these ESD diodes. This leakage is a function of the tem-
perature and common mode voltage of the amplifi er, as
shown in the Typical Performance Curves.
Noise
The LTC6241 exhibits exceptionally low 1/f noise in the
0.1Hz to 10Hz region. This 550nV
op amps to be used in a wide variety of high impedance
low frequency applications, where Zero-Drift amplifi ers
might be inappropriate due to their charge injection.
In the frequency region above 1kHz the LTC6241 also
show good noise voltage performance. In this frequency
region, noise can easily be dominated by the total source
resistance of the particular application. Specifi cally, these
amplifi ers exhibit the noise of a 3.1kΩ resistor, meaning it
is desirable to keep the source and feedback resistance at
or below this value, i.e. R
total source impedance, the noise voltage is not dominated
by the amplifi er.
Noise current can be estimated from the expression i
√2qI
and R√2qI
the amplifi er noise is dominated by the source resistance.
See the Typical Characteristics curve Noise Current vs
Frequency.
B
, where q = 1.6 • 10
B
Figure 2. Unity Gain Follower Test Circuit
Δf shows that for source resistors below 50GΩ
V
+2.5V
V
–2.5V
DD
SS
V
=
=
OUT
V
AND V
IN
IN
OF FOLLOWER WITH LARGE INPUT OVERDRIVE
R
S
–19
S
+ R
+
–
coulombs. Equating √4kTRΔf
LTC6241
+2.5V
–2.5V
1/2
G
||R
FB
P-P
6241 F02
≤ 3.1kΩ. Above this
noise allows these
V
OUT
62412f
n
=