LTC6241 LINER [Linear Technology], LTC6241 Datasheet - Page 16

• 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
Achieving Low Input Bias Current
The DD package is leadless and makes contact to the PCB
beneath the package. Solder fl ux used during the attach-
ment of the part to the PCB can create leakage current
paths and can degrade the input bias current performance
of the part. All inputs are susceptible because the backside
paddle is connected to V
changes or if V
and alter the observed input bias current. For lowest bias
current, use the LTC6241 in the SO-8 and provide a guard
ring around the inputs that are tied to a potential near the
input voltage.
A Digitally Programmable AC Difference Amplifi er
The LTC6241 confi gured as a difference amplifi er, can
be combined with a programmable gain amplifi er (PGA)
to obtain a low noise high speed programmable differ-
ence amplifi er. Figure 6 shows the LTC6241 based as a
single-supply AC amplifi er. One LTC6241 op amp is used
at the circuit’s input as a standard four resistor difference
amplifi er. The low bias current and current noise of the
LTC6241 allow the use of high valued input resistors, 100k
or greater. Resistors R1, R2, R3 and R4 are equal and the
LTC6241/LTC6242
phase lead to compensate the phase lag of the second
amplifi er. The op amp’s good input offset voltage match
and low input bias current means that the typical differential
output voltage is less than 40µV. A noise spectrum plot of
the differential output is shown in Figure 5.
16
140
120
100
80
60
40
20
0
Figure 5. Differential Output Noise
0
V
T
–3dB BW = 80kHz
–
S
A
= 25°C
10
= ±2.5V
changes, a leakage path can be formed
20
U
30
FREQUENCY (kHz)
–
40
U
internally. As the input voltage
50
60
70
W
80
90 100
6241 F05
U
V1
V2
gain of the difference amplifi er is one. An LTC6910-2 PGA
amplifi es the difference amplifi er output with inverting
gains of –1, –2, –4, –8, –16, –32 and –64. The second
LTC6241 op amp is used as an integrator to set the DC
output voltage equal to the LT6650 reference voltage V
The integrator drives the PGA analog ground to provide
a feedback loop, in addition to blocking any DC voltage
through the PGA. The reference voltage of the LT6650
can be set to a voltage from 400mV to V
resistors R5 and R6. If R6 is 20k or less, the error due
to the LT6650 op amp bias current is negligible. The low
voltage offset and drift of the LTC6241 integrator will not
contribute any signifi cant error to the LT6650 reference
voltage. The LT6650 V
20k
R1
R2
R6
C2
Figure 6. Wideband Difference Amplifi er with High
Input Impedance and Digitally Programmable Gain
DIGITAL INPUTS
G2
0
0
0
0
1
1
1
1
1
2
3
R1 = R2 = R3 = R4
+
–
G1
LTC6241
0
0
1
1
0
0
1
1
LT6650
1000pF
1/2
R3
R4
R5
C1
GO
0
1
0
1
0
1
0
1
GAIN
5
4
–16
–32
–64
–1
–2
–4
–8
0
REF
0.1µF
V
1µF
1µF
+
R
V
V
–
f
HIGH
REF
5 10
3
OUT
voltage has a maximum error
d BANDWIDTH
=
1k
=
=
= (V1 – V2) GAIN + V
0 4
k
2
. •
•
• •
(
π
8
OUT
1
V
5
⎛
⎝ ⎜
+
•
1
R
R
R
V
3
5
6
REF
LTC6910-2
•
AGND
+
7
2
C
=
G2 G1 G0
1
–
1
⎞
⎠ ⎟
(
0.1µF
2
f
100Ω
HIGH
)
f
6
3
LOW
IN
R6 20
+
–
REF
=
=
5
4
f
– 350mV with
LTC6241
V
LOW
2
C3
–
• •
1/2
k
π
)
GAIN
R
7
+
–
•
V
C
+
3
R7
62412f
REF
6241 F06
V
V
OUT
REF
.