DESIGN IDEAS L 40nVP–P Noise, 0.05µV/°C Drift, by Jim Williams Chopped FET Amplifier Figure 1’s circuit combines the 5V rail-to-rail performance of the LTC6241 with a pair of extremely low noise JFETs configured in a chopper based carrier modulation scheme to achieve extraordinarily low noise and DC drift. This circuit’s performance suits the demanding transducer signal conditioning situations such as high resolution scales and magnetic search coils. The LTC1799’s output is divided down to form a 2-phase 925Hz square wave clock. This frequency, harmonically unrelated to 60Hz, provides excellent immunity to harmonic beating or mixing effects which could cause instabilities. S1 and S2 receive complementary drive, causing the A1based stage to see a chopped version of the input voltage. A1’s square wave output is synchronously demodulated by S3 and S4. Because these switches are synchronously driven with the input chopper, proper amplitude and polarity information is presented to DC output amplifier A2. This stage integrates the square wave into a DC voltage, providing the output. The output is divided down (R2 and R1) and fed back to the input chopper where it serves as a zero signal reference. Gain, in this case 1000, is set by the VERT = 20nV/DIV HORIZ = 5s/DIV Figure 2. Noise measures 40nVP–P in 50s sample period TO LTC201 V + PIN 5V 18.5kHz V+ 5V 74C74 ÷ 2 74C90 ÷ 10 DIV LTC1799 OUT RSET Q 925Hz 54.2k* TO Ø1 POINTS 5V Ø1 –5V TO LTC201 V – PIN 1µF + 1µF + 5V 5V R1-R2 ratio. Because the input stage is AC coupled, its DC errors do not affect overall circuit characteristics, resulting in the extremely low offset and drift noted. Figure 2, noise measured over a 50 second interval, shows 40nV in a 0.1Hz to 10Hz bandwidth.This low noise is attributed to the input JFET’s die size and current density. L Q TO Ø2 POINTS 8 INPUT 3k 909Ω** 909Ω** 6 7 S1 S2 0.47µF 1µF LSK389 Ø2 11 10 9 499Ω** Ø2 1µF A1 LTC6241HV + 1µF 1 – 10M –5V 2 100k S3 S4 3 240k 14 15 16 100k 10k 0.001µF Ø1 1µF * = 0.1% METAL FILM RESISTOR ** = 1% METAL FILM RESISTOR = LTC201 QUAD = LSK389 = LINEAR INTEGRATED SYSTEMS FREMONT, CA NOISE = 40nVP-P 0.1Hz TO 10Hz OFFSET = 1µV DRIFT = 0.05µV/°C R2 +1 GAIN = R1 OPEN-LOOP GAIN = 10 9 I BIAS = 150pA – A2 LTC6241HV OUTPUT + R2 10k R1 10Ω Figure 1. 40nV noise chopper amplifier Linear Technology Magazine • March 2006 39