ADA4350ARUZ-R7 datasheet(28/38 Pages) AD

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Data Sheet

ADA4350

Rev. B | Page 27 of 37

THEORY OF OPERATION

KELVIN SWITCHING TECHNIQUES

Traditional gain selectable amplifiers use analog switches in a

feedback loop to connect discrete external resistors and capacitors

to the inverting input by selecting the appropriate feedback

path. This approach introduces several errors due to the nonideal

nature of the analog switches in the loop. For example, the

on-resistance of the analog switch causes voltage and temperature

dependent gain errors, while the leakage current causes offset

errors, especially at high temperature. The Kelvin switching

technique solves this problem by introducing two switches in

each gain selection loop, one to connect the transimpedance/

op amp output to the feedback network, and the other to connect

the feedback network output to the downstream components.

Figure 54 shows a programmable gain transimpedance amplifier

with Kelvin switching.

VOUT

S2B

HIGH IMPEDANCE LOAD

EXAMPLE

S1B

S2A

S1A

CF1

RF1

CF2

RF2

IPHOTO

NOTES

1. S1A, S1B, S2A, AND S2B ARE THE ANALOG SWITCHES.

RFx ARE THE FEEDBACK RESISTORS SPECIFIC TO EACH

TRANSIMPEDANCE PATH. CFx ARE THE FEEDBACK CAPACITORS

SPECIFIC TO EACH TRANSIMPEDANCE PATH.

Figure 54. Programmable Gain Transimpedance Amplifier with Kelvin

Switching

Although this technique requires using twice as many switches,

the voltage (Vx) in the center node is no longer switch dependent;

it is only dependent on the current across the selected resistor

(see Equation 1 through Equation 3).

VOUT = −IPHOTO × (RF2 + RS1B)

(1)

V1 = VOUT × (RF2/(RF2 + RS1B))

(2)

Substituting Equation 1 into Equation 2,

V1 = −IPHOTO × RF2

(3)

where:

VOUT is the output of the first amplifier.

IPHOTO is the current from the photodiode.

RF2 is the feedback resistor of Transimpedance Path 2.

RS1B is the switch resistance of the S1B switch.

The switches shown on the right (S2A and S2B) in Figure 54

only have a small output impedance and contribute negligible

error if the amplifier drives a high impedance load. In the case

of the ADA4350, the high impedance load is the integrated

ADC driver.

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