Upgrade Your Microcontroller ADC to True 12-Bit Performance Design Note DN463 by Guy Hoover Introduction Many 8-bit and 16-bit microcontrollers feature 10-bit internal ADCs. A few include 12-bit ADCs, but these often have poor or nonexistent AC specifications, and certainly lack the performance to meet the needs of an increasing number of applications. The LTC ®2366 and its slower speed versions offer a high performance alternative, as shown in the AC specifications in Table 1. Compare these guaranteed specifications with the ADC built into your current microcontroller. This family’s DC specifications are equally impressive. INL and DNL are guaranteed to be less than ±1LSB. Operating from a single 2.5V, 3V or 3.3V supply, the current draw on these parts is a maximum of 4mA during a conversion. This can be reduced to less than 1μA by placing the part into SLEEP mode during periods of inactivity, which greatly reduces the average supply current at lower sample rates. C3 0.1μF These ADCs are available in tiny 6-lead and 8-lead TSOT23 packages. The 8-lead devices have adjustable VREF and OVDD pins. The adjustable VREF pin allows the input span to be reduced to 1.4V. This, combined with the high ADC input impedance, can eliminate the need for gain or buffer stages in many applications. The OVDD pin, which controls the digital output level, can be adjusted from 1V to 3.6V, simplifying communication with different logic families. For applications that do not require an adjustable reference or adjustable output levels, the 6-lead device with VREF = OVDD = VDD should suffice. The SPI interface requires only three wires to communicate with the microcontroller, keeping the overall solution size small in low power, high speed applications. L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 0 3.3V 8192 Pt FFT fS = 2.84MHz VIN = –1dB AT 20kHz SNR = 72.1dB THD = –70.1dB R2 1k R4 499Ω AGND AIN R1 1k C4 10μF AGND + AGND LTC2366 R5 24.9Ω – AGND AV =–4 CS AIN LT6202 C2 1μF 1 2 5 VDD VREF OVDD R3 2.0k SCK SDO GND C1 0.0047μF 3 AGND dn463 F01 AGND Figure 1. Single Supply AC-Coupled Amplifier Level Shifts Input for Maximum Dynamic Range 8 7 6 FOURIER TRANSFORM –20 C5 0.1μF –40 –60 –80 –100 –120 –140 0 0.2 0.4 0.6 0.8 1.0 FREQUENCY (MHz) 1.2 1.4 dn463 F02 Figure 2. FFT Shows Low Noise and Distortion of Figure 1 Circuit 04/09/463 Application Circuits Figure 1 shows a single supply AC-coupled amplifier driving the LTC2366. This circuit is useful in applications where the sensor output level is too low to achieve full SNR performance from the ADC. The output of the LT®6202 swings rail-to-rail. This feature maximizes the circuit’s dynamic range when the op amp output is level shifted to the center of the ADC’s swing. The FFT of Figure 2 demonstrates the low noise and distortion of this circuit. the LTC6915 is a good choice for high gain applications. This circuit is useful for very low level signals or for applications with a wide range of input levels. Conclusion The 12-bit ADCs in the LTC236x family guarantee AC specifications that most built-in microcontroller ADCs cannot meet, thus improving performance when used in place of on-chip ADCs. The LTC236x family is easily interfaced to most microcontrollers via its SPI interface. A wide range of sample rates, an external reference pin and a separate OVDD pin provide additional flexibility. In Figure 3, a single supply DC amplifier with a programmable gain of 0 to 4096 drives the LTC2360. With a maximum offset of 10μV and a DC to 10Hz noise of 2.5μVP–P , Table 1. LTC236x ADC Family AC Specifications FULL LINEAR BANDWIDTH PART NUMBER SAMPLE RATE SINAD SNR THD LTC2366 3Msps 68dB (Min) 69dB (Min) –72dB (Max) 2.5MHz (Typ) LTC2365 1Msps 68dB (Min) 70dB (Min) –72dB (Max) 2.0MHz (Typ) LTC2362 500ksps 72dB (Typ) 73dB (Typ) –85dB (Typ) 1.0MHz (Typ) LTC2361 250ksps 72dB (Typ) 73dB (Typ) –85dB (Typ) 1.0MHz (Typ) LTC2360 100ksps 72dB (Typ) 73dB (Typ) –85dB (Typ) 1.0MHz (Typ) D3 D2 D1 D0 GAIN 0 0 0 0 0 0 0 0 1 1 0 0 1 0 2 0 0 1 1 4 0 1 0 0 8 0 1 0 1 16 0 1 1 0 32 0 1 1 1 64 1 0 0 0 128 1 0 0 1 256 1 0 1 0 512 1 0 1 1 1024 1 1 0 0 2048 1 1 0 1 4096 1 1 1 0 4096 1 1 1 1 4096 3.3V C10 0.1μF AIN P/SL + D0 D1 D2 D3 + LTC6915 V – V – R10 AGND 24.9Ω REF C9 10μF AGND 4 C6 300pF DGND 1 2 VDD VREF AIN LTC2360 5 OVDD CSL SCK SDO AGND 7 6 GND 3 AGND 8 dn463 F03 AGND AGND Figure 3. Single Supply DC Amplifier Provides Programmable Gain from 0 to 4096 Data Sheet Download www.linear.com For applications help, call (408) 432-1900, Ext. 2602 Linear Technology Corporation dn463 LT/TP 0409 155K • PRINTED IN THE USA FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2009 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ●