AD8628 datasheet(21/29 Pages) AD

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Data Sheet

ADA4523-1

Rev. 0 | Page 21 of 29

0.1

0.01

100k

FREQUENCY (Hz)

0.1

100

10k

VS = ±15V

CEXT = 0pF

CEXT = 50pF

Figure 67. Input Referred Current Noise Density vs. Frequency for CEXT = 0 pF

and CEXT = 50 pF

Another function of the input capacitance is to reduce the

effects of charge injection. The charge injection-based current

has frequency components at the 330 kHz chopping frequency

and its harmonics. In the time domain, these frequency

components appear as current pulses (appearing at regular

intervals related to the chopping frequency). When these small

current pulses interact with source impedances or gain setting

resistors, the resulting voltage spikes are amplified by the

closed-loop gain.

For higher source impedances, this may cause the 330 kHz

chopping frequency to be visible in the output spectrum, which

is known as clock feedthrough. To prevent excessive clock feed-

through, keep the gain setting resistors and source impedances

as low as possible. When dc highly resistive source impedance is

required, the capacitor across the source impedance reduces the

ac impedance, reducing the amplitude of the input voltage

spikes. Another way to reduce clock injection effects is to

bandwidth limit after the op amp output.

Injection currents from the two inputs are of equal magnitude

but opposite direction. Therefore, when chopping behavior is

the predominant source of IB, the effects of IB on the offset

voltage cannot be canceled by placing matched impedances at

both inputs.

Above 50°C, ESD protection diode leakage current begins to

dominate the input bias current and continues to increase

exponentially at elevated temperatures. The input bias cancellation

circuit of the ADA4523-1 minimizes this temperature driven

growth of the leakage current to keep the input bias current low

over all temperatures. Unlike injection current, leakage currents

are in the same direction for both inputs. Therefore, the output

error due to leakage current may be mitigated by matching the

source impedances seen by the two inputs. If the source impedance

matching technique is employed to cancel the effect of the

leakage currents, at less than 50°C, there is an offset voltage

error of 2IB × R due to the charge injection currents. For

example, if IB = 100 pA and R = 10 kΩ, the error is 2 μV.

THERMOCOUPLE EFFECTS

To achieve accuracy on the microvolt level, consider thermo-

couple effects. Any connection of dissimilar metals forms a

thermoelectric junction and generates a small temperature

dependent voltage, which is known as the Seebeck effect. These

thermal electric magnetic fields (EMFs) can be the dominant

error source in low drift circuits.

Connectors, switches, relay contacts, sockets, resistors, and

solders are all candidates for significant thermal EMF generation.

Even junctions of copper wire from different manufacturers can

generate thermal EMFs of 200 nV/°C, which is over 10 times

the maximum drift specification of the ADA4523-1. Figure 68

and Figure 69 illustrate the potential magnitude of these

voltages and their sensitivity to temperature.

3.0

2.8

2.6

2.4

2.2

1.8

2.0

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

TEMPERATURE (°C)

Figure 68. Thermal EMF Generated by the Junction of Two Copper Wires

from Different Manufacturers

100

–50

–100

1020

304050

SOLDER TO COPPER JUNCTION DIFFERENTIAL TEMPERATURE

SOURCE: NEW ELECTRONICS 02-06-77

SLOPE ≈ 1.5µV/°C

BELOW 25°C

SLOPE ≈ 160nV/°C

BELOW 25°C

64% Sn/36% Pb

60% Cd/40% Sn

Figure 69. Solder Copper Thermal EMFs

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