OPA2191: Clarification of datasheet for OPA2191 and OPA2145

Hello Jonathan,

Regarding your questions about the OPA191:

Q1: Do I need the snubber resistor and cap for my case:
    single 9V battery supply
    inverting with unity or near unity gain
    load is 25K ohm volume pot -> cable with up to 1nF capacitance -> 1 meg ohm amplifier input impedance.

Figure 8-5. Transient Response in the OPA191 datasheet show the output with a Purely Capacitive Load of 1 nF. There is small amount of overshoot indicating that there is sufficient phase margin and the circuit is stable. Since you load is a parallel combination of about 25 kilohms in parallel with 1 nF the output load does include a shunt resistance to ground and this should further improve the phase margin by a few degrees. I ran some quick simulations of the OPA191 connected as a unity gain buffer with a 1 nF load with and without the snubber and the transient response was nearly identical. It shouldn't require the snubber.

Q2: If I substitute OPA2145 for OPA2191 (both SOIC-8) should I include the snubber or delete it?

Viewing OPA2145 datasheet Figure 6-27, Small-Signal Overshoot vs Capacitive Load, it appears that in a unity gain configuration it will only tolerate a few hundred picofarad load before overshoot becomes excessive. That indicates the phase margin is becoming much reduced and instability will occur with a larger capacitive load likely including 1 nF. The OPA191 snubber circuit is optimized for the OPA191 AC electrical characteristics and although it may be effective with the OPA2145, it may not. If the circuit can tolerate a few ohms of added resistance (an Riso) in series with the OPA2145 output that goes a long way in increasing the phase margin and assuring stability. 

Datasheet for OPA145/2145 suggests a Zener diode clamp for positive supply (Vcc).

Q3: Should I include a current-limiting resistor with it?
    Vcc = 9V or 18VDC nominal
    Zener 500mW or 1W 20V to ground at the supply, e.g. onsemi 1SMA5932BT3G
    1W @ 20V = 50mA, 20V / 50mA = 400 ohms

If your circuit has a clean, regulated supply for V+ and a well established in specification input voltage it should not be necessary to add a Zener, or transient voltage suppressor (TVS) diode to the supply line. That is suggested when the supply characteristics are not well regulated and might overshoot the maximum supply rating of the OPA2145 (40 V), or if the op amp may be subjected to an input electrical over-stress (EOS) condition. If you do decide to include a TVS at V+, then the diode voltage should be selected such that it doesn't turn on unless the V+ value is exceeded by a few volts. Likely a 2 V margin or bit more would suffice.

Regards, Thomas

Precision Amplifiers Applications Engineering