LTC1250CS8#TR Linear Technology, LTC1250CS8#TR Datasheet - Page 6

• Manufacturer Part Number: LTC1250CS8#TR
• Description: IC AMP BRIDGE ZERO DRFT LN 8SOIC
• Manufacturer: Linear Technology

Specifications of LTC1250CS8#TR

• Amplifier Type: Chopper (Zero-Drift)
• Number Of Circuits: 1
• Output Type: Rail-to-Rail
• Slew Rate: 10 V/µs
• Gain Bandwidth Product: 1.5MHz
• Current - Input Bias: 50pA
• Voltage - Input Offset: 5µV
• Current - Supply: 3mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 16 V, ±2.38 V ~ 8 V
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (3.9mm Width)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Current - Output / Channel: -
• -3db Bandwidth: -

LTC1250
A
Input Noise
The LTC1250, like all CMOS amplifiers, exhibits two types
of low frequency noise: thermal noise and 1/f noise. The
LTC1250 uses several design modifications to minimize
these noise sources. Thermal noise is minimized by rais-
ing the g
high bias levels and using large transistor geometries. 1/f
noise is combated by optimizing the zero-drift nulling loop
to run at twice the 1/f corner frequency, allowing it to
reduce the inherently high CMOS 1/f noise to near thermal
levels at low frequencies. The resultant noise spectrum is
quite low at frequencies below the internal 5kHz clock
frequency, approaching the best bipolar op amps at 10Hz
and surpassing them below 1Hz (Figure 1). All this is
accomplished in an industry-standard pinout; the LTC1250
requires no external capacitors, no nulling or clock sig-
nals, and conforms to industry-standard 8-pin DIP and
8-pin SO packages.
Input Capacitance and Compensation
The large input transistors create a parasitic 55pF capaci-
tance from each input to V
react with the external feedback resistors to form a pole
which can affect amplifier stability. In low gain, high
impedance configurations, the pole can land below the
unity-gain frequency of the feedback network and degrade
phase margin, causing ringing, oscillation, and other
unpleasantness. This is true of any op amp, however, the
55pF capacitance at the LTC1250’s inputs can affect
6
PPLICATI
M
of the front-end transistors by running them at
80
70
60
50
40
30
20
10
Figure 1. Voltage Noise vs Frequency
0
0.01
LTC1250
O
U
OP-27
S
FREQUENCY (Hz)
I FOR ATIO
U
+
0.1
OP-07
. This input capacitance will
V
R
W
S
S
= ±5V
= 10Ω
LTC1250 F01
1
U
stability with a feedback network impedance as low as
1.9k. This effect can be eliminated by adding a capacitor
across the feedback resistor, adding a zero which cancels
the input pole (Figure 2). The value of this capacitor should
be:
where A
on the value of R
low loop impedance should not require C
Larger values of C
circuits, may actually increase low frequency noise. The
nulling circuitry in the LTC1250 closes a loop that includes
the external feedback network during part of its cycle. This
loop must settle to its final value within 150µs or it will not
fully cancel the 1/f noise spectrum and the low frequency
noise of the part will rise. If the loop is underdamped (large
R
offset will suffer.
The solution is to add C
a value of C
from reaching a final value by the 150µs deadline. This
condition doesn’t affect the LTC1250’s offset or output
stability, but 1/f noise begins to rise. As a rule of thumb,
the R
frequency at which the loop settles) for best 1/f perfor-
mance; values between 100pF and 500pF work well with
feedback resistors below 100k. This ensures adequate
gain at 7kHz for the LTC1250 to properly null. High value
feedback resistors (above 1M) may require experimenta-
tion to find the correct value because parasitics, both in the
C
F
F
, no C
Figure 2. C
≥
F
55
C
A
F
F
V
pF
) it will ring for more than 150µs and the noise and
V
= closed-loop gain. Note that C
feedback pole should be ≥ 7kHz (1/150µs, the
R
IN
F
C
F
P
will overdamp the loop, again preventing it
Cancels Phase Shift Due to Parasitic C
F
. Circuits with higher gain (A
F
, commonly used in band-limited DC
–
+
LTC1250
F
C
R
as above but beware! Too large
F
F
1250 F02
F
is not dependent
F
for stability.
V
P
> 50) or
1250fb