LTC6240/LTC6241/LTC6242
19
624012fe
Amplifi er Characteristics
Figure 1 is a simplifi ed schematic of the amplifi er, 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
–
, no phase inversion occurs. For
input signals greater than a diode drop below V
–
, limit the
current to 3mA with a series resistor R
S
to avoid phase
inversion.
ESD
The LTC6240/LTC6241/LTC6242 have 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.
The amplifi er input bias current is the leakage current of
these ESD diodes. This leakage is a function of the tempera-
ture and common mode voltage of the amplifi er, as shown
in the Typical Performance Characteristics curves.
Noise
The LTC6240/LTC6241/LTC6242 exhibit exceptionally
low 1/f noise in the 0.1Hz to 10Hz region. This 550nV
P-P
noise allows these 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 LTC6240/LTC6241/
LTC6242 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 3.1k
resistor, meaning it is desirable to keep the source and
feedback resistance at or below this value, i.e. R
S
+ R
G
||R
FB
≤ 3.1k. Above this total source impedance, the noise
voltage is not dominated by the amplifi er.
Noise current can be estimated from the expression i
n
=
√2qI
B
, where q = 1.6 • 10
–19
coulombs. Equating √4kTRΔf
and R√2qI
B
Δf shows that for source resistors below 50G
the amplifi er noise is dominated by the source resistance.
See the Typical Performance Characteristics curve Noise
Current vs Frequency.
Figure 1. Simplifi ed Schematic Figure 2. Unity Gain Follower Test Circuit
APPLICATIONS INFORMATION
R2
Q2
6241 F01
V
IN
+
I
TAIL
V
IN
–
V
O
V
+
V
+
V
–
V
–
V
–
CM
DESD5
DIFFERENTIAL
DRIVE
GENERATOR
BIAS
DESD6
V
+
DESD2
V
+
DESD4
V
–
DESD1
V
–
DESD3
R1
Q1
M2M1
M3
M4
C1
CLAMP
+2.5V
–2.5V
6241 F02
–
+
LTC6240
R
S
V
IN
V
OUT
V
OUT
AND V
IN
OF FOLLOWER WITH LARGE INPUT OVERDRIVE
V
DD
=
+2.5V
V
SS
=
–2.5V
