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Rev C, 4/27/2023

Page 23

© TT electronics plc

FlexSense® I-Series Programmable

Incremental Encoder

FS210/FS310 Datasheet

Signal Conditioning

For ideal system-level determination of position and velocity, the sinusoidal quadrature-track analog voltages produced

from the aggregated photocurrents from the FS210/FS310’s properly-configured pixel array should have zero DC offset and

equal amplitudes and be exactly 90° out of phase. Deviations from these ideal signal characteristics can arise from several

sources, such as:

• Mismatch between the LED’s emission pattern, the code wheel, and the PXC/CCFG pattern

• Imperfections in code-wheel printing

• Code-wheel eccentricity and/or presence of an out-of-plane component of rotation

• Transistor-level mismatches in the FlexSenseTM chip

The FS210/FS310 contains registers to trim the quadrature-track DC offsets and amplitudes. below illustrates the

quadrature-track signal path. The nominal value of the TIA feedback resistors is 230kΩ if the TLNM bit in the ZCFG control

register is 0, or 700kΩ if the TLNM bit is 1. The power-on default voltage gain of the second-stage buffer is 4.

90°

A+ (Positive COS)

A- (Negative COS)

B+ (Positive SINE)

B- (Negative SINE)

xG

TIA

xG

TIA

Current

Voltage

Optical

A+

A-

B+

B-

TQM2

TQM1

TDCA/B

Figure 10: Differential Photocurrents (A+/A- & B+/B-) Signal paths

A small current can be injected in parallel with either the A+ or the A- aggregated photocurrent, and another small current

can be injected in parallel with either the B+ or the B- aggregated photocurrent. The TDCA and TDCB trim registers,

respectively, control these currents. Adding trim current in parallel with the A+ (B+) aggregated photocurrent acts to make

the DC offset of the A (B) differential output voltage more positive or less negative. Adding trim current in parallel with the

A- (B-) aggregated photocurrent acts to make the DC offset of the A(B) differential output voltage less positive or more

negative. Up to 31 steps of 4nA each can be added to either the A+ or the A- aggregated photocurrent, and up to 31 steps

of 4nA each can be added to either the B+ or the B- aggregated photocurrent.

Mismatches in the amplitudes of the A+/A- and B+/B- differential output voltages can be counteracted by reducing the

feedback transresistances around whichever of the A or B transimpedance amplifier produces a greater untrimmed

differential sinusoidal voltage amplitude. The TQM1 trim register controls this transimpedance attenuation: one bit selects

either the A or B transresistances to be attenuated, and the remainder of the register allows the selected transresistances

to be reduced by up to 7 steps of 5% each.