LTC2057_15 Datasheet, PDF (18/32 Pages) Linear Technology – High Voltage, Low Noise Zero-Drift Operational Amplifier

LTC2057/LTC2057HV
Applications Information
Input Voltage Noise
Chopper stabilized amplifiers like the LTC2057 achieve low
offset and 1/f noise by heterodyning DC and flicker noise
to higher frequencies. In a classical chopper stabilized
amplifier, this process results in idle tones at the chopping
frequency and its odd harmonics.
The LTC2057 utilizes circuitry to suppress these spurious
artifacts to well below the offset voltage. The typical ripple
magnitude at 100kHz is much less than 1µV_RMS.
The voltage noise spectrum of the LTC2057 is shown in
Figure 1. If lower noise is required, consider one of the
following circuits from the Typical Applications section:
"DC Stabilized, Ultralow Noise Amplifier" or "Paralleling
Choppers to Improve Noise."
35 _{AV = +11}
30 ^{VS = ±2.5V}
25
20
15 _{NO 1/f NOISE}
10
5
0
0.1 1 10 100 1k 10k 100k 1M
FREQUENCY (Hz)
2057 F01
Figure 1. Input Voltage Noise Spectrum
Input Current Noise
For applications with high source impedances, input cur-
rent noise can be a significant contributor to total output
noise. For this reason, it is important to consider noise
current interaction with circuit elements placed at an
amplifier’s inputs.
The current noise spectrum of the LTC2057 is shown in
Figure 2. The characteristic curve shows no 1/f behavior.
As with all zero-drift amplifiers, there is a significant cur-
rent noise component at the offset-nulling frequency. This
phenomenon is discussed in the Input Bias Current section.
0.25
NO 1/f NOISE
0.20
A_V = +11
V_S = ±2.5
0.15
0.01
0.05
0
0.1
1
10
100
1k
10k
FREQUENCY (Hz)
2057 F02
Figure 2. Input Current Noise Spectrum
It is important to note that the current noise is not equal
to 2_qI_B. This formula is relevant for base current in bipolar
transistors and diode currents, but for most chopper and
auto-zero amplifiers with switched inputs, the dominant
current noise mechanism is not shot noise.
Input Bias Current
As illustrated in Figure 3, the LTC2057’s input bias current
originates from two distinct mechanisms. Below 75°C,
input bias current is nearly constant with temperature,
and is caused by charge injection from the clocked input
switches used in offset correction.
100 _{1 TYPICAL UNIT}
V_S = ±2.5V
10
1
25°C MAX I_B SPEC
0.1
0.01
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
2057 F03
Figure 3. Input Bias Current vs Temperature
2057f
18
For more information www.linear.com/LTC2057