LT6014 Linear Technology Corporation, LT6014 Datasheet - Page 12

• Manufacturer Part Number: LT6014
• Description: Single/dual 145?a, 9.5nv/rthz, Av >=5, Rail-to-rail Output Precision Op Amps
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT6013/LT6014
Not Unity-Gain Stable
The LT6013 and LT6014 amplifiers are optimized for the
lowest possible noise and smallest package size, and are
intentionally decompensated to be stable in a gain con-
figuration of 5 or greater. Do not connect the amplifiers in
a gain less than 5 (such as unity-gain). For a unity-gain
stable amplifier with similar performance though slightly
higher noise and lower bandwidth, see the LT6010 and
LT6011/LT6012 datasheets.
Figure 1 shows simple inverting and non-inverting op amp
configurations and indicates how to achieve a gain of 5 or
greater. For more general feedback networks, determine
the gain that the op amp “sees” as follows:
12
1. Suppose the op amp is removed from the circuit.
2. Apply a small-signal voltage at the output node of the
op amp.
V
REF
V
IN
R
U
INVERTING:
SIGNAL GAIN = –R
OP AMP GAIN = 1 + R
STABLE IF 1 + R
V
G
IN
10k
U
+
–
F
/R
2.5k
UNITY GAIN FOLLOWER
R
F
1nF
G
/R
F
≥ 5
F
G
Figure 1. Use LT6013 and LT6014 in a Gain of 5 or Greater
/R
W
G
Figure 2. Stabilizing Op Amp for Unity Gain Operation
+
–
V
IN
U
NONINVERTING:
SIGNAL GAIN = 1 + R
OP AMP GAIN = 1 + R
STABLE IF 1 + R
V
REF
V
R
OUT
G
+
–
F
/R
Do not place a capacitor bigger than 200pF between the
output to the inverting input unless there is a 5 times larger
capacitor from that input to AC ground. Otherwise, the op
amp gain would drop to less than 5 at high frequencies,
and the stability of the loop would be compromised.
The LT6013 and LT6014 can be used in lower gain
configurations when an impedance is connected between
the op amp inputs. Figure 2 shows inverting and non-
inverting unity gain connections. The R
the op amp inputs results in a large enough noise gain at
high frequencies, thereby ensuring stability. At low fre-
quencies, the capacitor is an open circuit so the DC
precision (offset and noise) remains very good.
R
G
F
V
F
≥ 5
F
IN
/R
3. Find the differential voltage that would appear across
4. The ratio of the output voltage to the input voltage is
/R
G
G
the two inputs of the op amp.
the gain that the op amp “sees”. This ratio must be
5 or greater.
10k
3k
UNITY GAIN INVERTER
1nF
V
IN
–
+
UNITY-GAIN:
DO NOT USE
10k
+
–
60134 F02
V
OUT
C
60134 F01
network across
60134fb