Video Post Processor

Stephen Kaye

2013-3-28

Preamp Design – Review

• Preamp based on instrumentation amp

• Preamp gain set to 2 V/V

• Input referred noise 40 nV/√Hz

• Impedance matched to 100 Ω lines

• Preamp delivers fully differential signal

Video Processor Design

DifferentialInput fromPreamp

Common Mode Feedback

DifferentialAmplifier

ADC

Filter Reset

Video Processor – Differential Amplifier

• Differential Buffer AD4938-1

• Two output terminals +OUT and –OUT

• Outputs are 180º out of phase

• Two feedback loops

• Input to set common mode voltage

Video Processor – Differential Amplifier

Video Processor – Differential Offset

Video Processor – CM Offset

• Common mode output set by Vocm input

• Common mode set by ADC to Vref/2

• Additional ADC adjustment available

Vocm Feedback

Video Processor – Noise

• Differential noise model– Voltage noise– Current noise– Resistor noise

• Well matched resistors eliminate common mode voltage noise

Video Processor - NoiseInput

Referred Noise

Multiplication Factor

Differential Input

2.6 nV/√Hz GN

Inverting Input

4.8 nV/√Hz GN

Noninverting Input

4.8 nV/√Hz GN

Vocm Input 7.5 nV/√Hz GN(β – β)

Gain Resistors

5.74 nV/√Hz GN(1 – β)

Feedback Resistors

5.74 nV/√Hz 1

Voltage Offset Resistors

5.74 pV/√Hz GN(1 – β)

• Resistor noise e = √4kTR

• Each input has multiplication factor for output

• GN = 2

• β = 0.5

Video Processor - NoiseOutput Noise

Differential Input 5.2 nV/√Hz

Inverting Input 9.6 nV/√Hz

Noninverting Input 9.6 nV/√Hz

Vocm Input 0 nV/√Hz

Gain Resistors 5.74 nV/√Hz

Feedback Resistors

5.74 nV/√Hz

Voltage Offset Resistors

5.74 pV/√Hz

• Output noise total is sum of squares of contributions

• Total output voltage noise 20.22 nV/√Hz

• Total output voltage noise from preamp 57.3 nV/√Hz

• Sum of noise is 60.76 nV/√Hz

Video Processor – Anti-Aliasing Filter

• Simple single pole RC filter

• ADC sampling frequency is 6MHz

• Anti-aliasing filter 3dβ set to 3 MHz

• Filter resistor noise is 0.46 nV/√Hz – negligible

• Filter reset by switch to ground with Ron = 1Ω

Video Processor – ADC Sampling

• 6x oversampling of 1 MHz pixel rate

• Three reset signal samples and three charge signal samples

• Samples are averaged for reset signal and charge signal– Effectively creates another filter pole at 3 MHz

• CDS performed using averaged reset signal and charge signal for each pixel