ADA4177-1 Datasheet, PDF (26/31 Pages) Analog Devices – OVP and EMI Protected, Precision, Low Noise and Bias Current Op Amps

ADA4177-1/ADA4177-2/ADA4177-4
J1A, J1B, J2A, and J2B are depletion mode junction field effect
transistors (JFETs) that replace the series resistance in the
conventional protection scheme. Under normal operation, the
input bias current of the ADA4177-1/ADA4177-2/ADA4177-4
flows through the J1A and J2A transistors without pinching off
the channel. To achieve excellent noise performance, J1A and
J2A must have a low on resistance (R_DSON) of approximately
300 Ω.
When either input exceeds the rail by more than a diode, large
currents flow through either J1A or J2A, which causes the
channels to pinch off and effectively raises their resistance.
Figure 88 shows the positive overvoltage and negative
overvoltage characteristics as the FET channel pinches.
12
10
8
6
V_SY = ±15V
4
2
0
–2
–4
–6
–8
–10
–50
–30
–10
10
30
50
V_IN (V)
Figure 88. Input Bias Current During Positive and Negative Overvoltage,
V_SY = ±15 V, Voltage Follower Configuration
Figure 89 shows how the JFET effective resistance increases
exponentially as shown by the measurements at 2 V, 20 V,
and 40 V overvoltage. Note that as the overvoltage increases
from 2 V to 40 V, the resistance increases from 300 Ω to 3.5 kΩ
(a factor of 11).
10
300Ω AT 2V
8
2.2kΩ
AT 20V
3.5kΩ AT 40V
6
4
2
0
–2
2.5
7.5 12.5 17.5 22.5 27.5 32.5 37.5 42.5
V_IN (V)
Figure 89. Overvoltage vs. Input Voltage (V_IN), Voltage Follower
Configuration
Data Sheet
LIMITING OVERVOLTAGE CURRENT OUT OF THE
POSITIVE SUPPLY PIN
Because the positive power supply of the system may be incapable
of sinking the large overvoltage current of 8 mA (see Figure 88),
care was taken to divide down this current into the positive rail
during an overvoltage event. As shown in Figure 90, Q1L is a
lateral PNP transistor that serves two purposes. First, the
emitter base acts as a clamping diode to route the overvoltage
current away from the V+ pin and to the V− pin. Second, it
divides down this current via the beta of Q1L. At an emitter
current of 8 mA, the beta of Q1L is approximately 8, which
reduces the current injected into the positive supply by a factor
of 8.
V+
J1B
Q1L
+IN
J1A
V–
Figure 90. Overvoltage Protection Circuitry
Figure 91 shows the positive and negative supply currents when
the input voltage exceeds the supply voltages (and overvoltage
condition). The current at the V+ terminal does not reverse
direction during an overvoltage event because the current is
directed to V− via the collector of Q1L.
8
6
4
POSITIVE SUPPLY CURRENT
2
0
–2
–4
–6
–8
NEGATIVE SUPPLY CURRENT
–10
–12
–40 –30 –20 –10
0
10
20
30
40
INPUT DIFFERENTIAL (V)
Figure 91. Supply Current vs. Input Differential, Circuit Configured at
Unity Gain with V+ = +15 V and V− = −15 V
If negative overvoltage transients are expected, ensure that the
negative voltage source driving V− can handle sourcing current
without forcing current into the device and causing the supply
voltage to change.
Rev. C | Page 26 of 31