Basic Building Blocks for Analog/Mixed Signal ICs

Bandgap Reference

Performance Metrics

• Absolute accuracy: 20mV to 40mV for 1.2 Vref
• Temperature Coefficiency TC: 80ppm/C
• PSRR: 80dB @ 10KHz, 40dB @ 500KHz,
• Area: 400umX200um
• Power consumption: 1 mW to 1.5 mW

Case Study

Non-ideal Effects

• The offset of the OTA Vos. The input refered offset voltage of the OTA is about 2mV. Then we have:
  Vbg=Vbg(ideal)+R2/R3*(Vos)
  Assume R2/R3=10, we have: Vbg=Vbg(ideal)+20mV.
  We know Vbg is about 1.2V, so Vos introduce 20mV/1.2V=1.6% error.
  Assume ideal case the temperature coefficiency TC_F is 0 at room temperature, then this offset introduce TC_F of about: 20mV/(1.2V*300)=55ppm/K
• current desensity (BJT area/current in two branch) mismatch:Assume there is 10% mismatch between the two diodes, it introduces (R2/R3)*(V_Tln(1.1m))-(R2/R3)*(V_Tlnm). Assume lnm=ln8, R2/R3=10, we have: DVbg=10*26mV*(ln8.8-ln8)=24mV. Now assume 1% mismatch, DVbg=10*26mV*(ln8.08-ln8)=2.5mV
• Resistor mismatch: mismatch between R2 and R3, assume 1% mismatch, DVbg=(10.01-10)*26mV*(ln8)=0.5mV, or DVbg=(10.01-10)*26mV*(ln24)=0.8mV

We can see that the dominant nonideal effect is the offset as well as low frequency 1/f flicker noise of the opamp. One method to reduce the offset and 1/f noise is to use a chopper-stabilized opamp instead of a normal opamp. Here is an example using chopper-stabilized opamp(PDF) by M.A.T. Sanduleanu in Electronics Letters, May 1998

Special Cases

The traditional bandgap voltage is about 1.25 V. But some application need higher references, such as ADC. For example, for a ADC operates at 3V power supply, the required reference may be 2V. Another case is that as the technology scale, power supply voltage scale down too. For 90nm process, the powr supply is less than 1.2V, which is lower than the bandgap voltage.

• Higher bandgap voltage references (>1.2V)
• Low power supply (<1.2V) bandgap voltage references A presentation on sub-1v bandgap reference design (PDF)

Opamp Design

Arizona State University EEE523: Advanced Analog Circuit Design documents:

• A compact folded-cascode class-AB amplifier (PDF). In this course project, I designed a high portable, robust class-AB amplifier. It's low-frequency gain is about 96 dB with a unit gain-bandwidth larger than 50MHz. This is a very power and area efficient design. I really like it. It is very easy to be modified to meet different applications. It features wide-swing constant-gm bias circuitry.
  But this design is not ready for type-out yet. Why? ha....you tell me
  
  
• A home work write-up (PDF).



• Report of project #1 (PDF)

Here is a presentation on how to characterizatize the differential amplifier (from Cadence, PDF)