Download Input Diodes for ac Coupled Circuits

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.