AD8628 datasheet(20/29 Pages) AD

20 / 29 page

ADA4523-1

Data Sheet

Rev. 0 | Page 20 of 29

THEORY OF OPERATION

The block diagram and clamp circuits are shown in Figure 64.

+IN

–IN

SDCOM

OUT

V–

V+

100Ω

D10

D13

D_ESD

100Ω

LOGIC

0.5µA

2.5µA

V+

V–

D11

D12

V+

V–

10kΩ

–

Figure 64. Block Diagram and Clamp Circuits (SD Is the Internal Shutdown)

INPUT VOLTAGE NOISE

Chopper stabilized amplifiers, such as the ADA4523-1, achieve

low offset and 1/f noise by heterodyning dc and flicker noise to

higher frequencies. In a typical chopper stabilized amplifier, this

process results in idle tones at the chopping frequency and its

odd harmonics.

The ADA4523-1 uses circuitry to suppress these spurious artifacts

to below the offset voltage. The typical ripple magnitude at the

330 kHz chopping frequency is less than 1 µV rms.

The input voltage noise spectral density of the ADA4523-1 is

shown in Figure 20. If lower noise is required, see Figure 75 in

the Applications Information section.

INPUT CURRENT NOISE

For applications with high source impedances, input current

noise can be a significant contributor to the total output noise.

For this reason, it is important to consider noise current

interaction with circuit elements placed at the inputs of the

amplifier.

The input current noise spectral density of the ADA4523-1 is

shown in Figure 22 and as measured by the circuit in Figure 65,

with a shunt capacitance (CEXT) = 0 pF. The characteristic curve

shows no 1/f behavior. As with all zero drift amplifiers, there is a

significant current noise component at the offset nulling

frequency, which is discussed further in the Input Bias Current

section.

It is important to note that the current noise is not equal to 2qIB,

where q is the charge of an electron, 1.6 × 10−19Coulombs.

This formula is relevant for base current in bipolar transistors

and diode currents. However, for most chopper and autozero

amplifiers with switched inputs, the dominant current noise

mechanism is not shot noise.

1MΩ

CEXT

TEST CIRCUIT

–

Figure 65. Input Current Noise Spectrum Test Circuit

INPUT BIAS CURRENT

The input bias current of the ADA4523-1 comprises two

different currents, leakage and charge injection. Leakage

current increases with temperature, while the charge injection

current from the switching input remains relatively constant

with temperature. The composite of these two currents over

temperature is shown in Figure 14.

How the various input bias currents behave and contribute to

error depends on the nature of the source impedance. For the

input bias currents specified in Table 1 and Table 2, the source

impedances are high value resistors bypassed with CEXT, in the

same configuration as shown in Figure 65. Figure 66 shows the

effective dc error as an input referred current error (output dc

voltage error divided by gain and then by the source resistance)

as a function of CEXT. Note that the effective dc error decreases

as the capacitance increases. The added CEXT also reduces the

input referred current noise density as shown in Figure 67.

250

100

500

CEXT (pF)

115

130

145

160

175

190

205

220

235

100

150

200

250

300

350

400

450

VS = ±15V

ONE RANDOMLY SELECTED INPUT

Figure 66. Effective IB vs. CEXT

AD8628 Datasheet(PDF) 20 Page - Analog Devices

AD8628 Datasheet(HTML) 20 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

Datasheet Download

Link URL