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Input Diodes for ac Coupled Circuits 12/31/2015 Why does the average input voltage rise? • Many op amps have back-to-back diodes across the input • During slew rate condition the amplifiers differential voltage may be volts • Volts across back-to-back diodes turns them on. This charges the input capacitor • A “saw tooth” type waveform will only slew on the fast edge. So it only slews in one direction. This will charge the input capacitor on each sharp edge. • Using op amps without back-to-back diodes will solve the issue OPA1642: Input • In your initial E2E post you mentioned that you see the problem with the OPA1642 • This did not make sense to us as the OPA1642 does not have back-to-back diodes, so this device should not have showed the issue. • Please confirm that you were using the OPA1642 and not the OPA1652 or another device. • The use of back-to-back diodes is common and so the OPA1642 is one of the best candidates for this circuit. • The response to your posting was delayed as I needed to order the OPA1642 to confirm that it did not show the issue you mentioned. I built the circuit and it behaved normally. What are back to back input diodes Simulate With and Without Input Diodes R1 1k Rf 1k Vfalling 0 AM1 Vc1 C1 10u + + SW-SPDT1 Vrising 0 Try with sharp rising edge or sharp falling edge + R3 499k A V2 15 V D2 1N1183 + D1 1N1183 SW-SPST1 VOUT ++ U2 OPA171 + V Vin_opa V1 15 Connect and disconnect input diodes With diodes, sharp falling edge T 5.26m AM1 -14.04m 14.50 VOUT -7.69 5.52 Notice that the voltage on the capacitor increases after start-up. Vc1 0.00 9.52 Vin_opa -4.00 The current coming out of the non-inverting input is miliamps! This current is from op amp slewing. T 0.00 50.00m Time (s) 99.50m 99.75m Time (s) 100.00m 5.26m AM1 -14.04m 14.50 VOUT -7.69 5.52 Vc1 0.00 9.52 ac coupled diodes.TSC Vin_opa -4.00 100.00m With diodes, sharp rising edge T 14.33m AM1 -5.26m 7.70 VOUT With rising edge the voltage on the capacitor now decreases after startup. It increased with falling edge. -14.73 0.00 Vc1 -5.65 4.00 Vin_opa -9.65 0.00 The polarity of the current pulses changed from falling edge T 50.00m Time (s) 100.00m 14.33m AM1 -5.26m 7.70 VOUT -14.73 0.00 Vc1 -5.65 4.00 Vin_opa -9.65 99.50m 99.75m Time (s) 100.00m Diodes Disconnected, sharp rising edge T 46.42u AM1 -10.06u 7.20 VOUT The voltage across the capacitor is practically constant at zero volts. -7.99 2.73u Vc1 -20.09u 4.00 Vin_opa -4.00 0.00 Current in to the noninverting input is very small. Capacitive coupling through input capacitor. T 46.42u 50.00m Time (s) 100.00m AM1 -10.06u 7.20 VOUT -7.99 2.73u Vc1 -20.09u 4.00 Vin_opa -4.00 99.50m 99.75m Time (s) 100.00m Measured results OPA277 – has diodes OPA277 OPA192 – no diodes OPA192 Measured results OPA1642 – no diodes OPA1642 – no diodes Conclusion • In the cases with back-to-back diodes the input coupling capacitor charges during the fast edge of the sawtooth waveform. • In the case of amplifiers without back-to-back diodes the input capacitor is not charged. • For this type of application you need to use amplifiers without back-to-back input diodes. • Note 1: Simulation and Measured results match. • Note 2: For devices with the diodes, the effect is minimized for amplifiers with high slew rate as the current charging the input capacitor is trunked on for a shorted time period. • Note 3: For devices with the diodes, the effect is minimized for symmetrical waveforms; i.e. if the amplifier slews equally on both directions, the charging and discharging will cancel. • Note 4: For devices with the diodes, you will only see this effect if the amplifier slews. In normal operation, you will not turn on the input diodes and will not see this charging.