OPA145 Datasheet, PDF (19/40 Pages) Texas Instruments – High-Precision, Low-Noise, Rail-to-Rail Output, 5.5-MHz JFET Operational Amplifiers

www.ti.com
OPA145, OPA2145, OPA4145
SBOS427 – JUNE 2017
8.3 Feature Description
8.3.1 Operating Voltage
The OPA145, OPA2145, and OPA4145 series of op amps can be used with single or dual supplies from an
operating range of V_S = 4.5 V (±2.25 V) up to V_S = 36 V (±18 V). These devices do not require symmetrical
supplies; they only require a minimum supply voltage of 4.5 V (±2.25 V). For V_S less than ±3.5 V, the common-
mode input range does not include midsupply. Supply voltages higher than 40 V can permanently damage the
device; see the Absolute Maximum Ratings table. Key parameters are specified over the operating temperature
range, T_A = –40°C to +125°C. Key parameters that vary over the supply voltage, temperature range, or
frequency are shown in Typical Characteristics.
8.3.2 Capacitive Load and Stability
The dynamic characteristics of the OPAx145 have been optimized for commonly encountered gains, loads, and
operating conditions. The combination of low closed-loop gain and high capacitive loads decreases the phase
margin of the amplifier and can lead to gain peaking or oscillations. As a result, heavier capacitive loads must be
isolated from the output. The simplest way to achieve this isolation is to add a small resistor (R_OUT equal to 50 Ω,
for example) in series with the output.
Figure 27 illustrates the effects on small-signal overshoot for several capacitive loads. Also, see Feedback Plots
Define Op Amp AC Performance, available for download from the TI website, for details of analysis techniques
and application circuits.
8.3.3 Output Current Limit
The output current of the OPAx145 series is limited by internal circuitry to +20 mA/–20 mA (sinking/sourcing), to
protect the device if the output is accidentally shorted. This short-circuit current depends on temperature, as
shown in Figure 36.
8.3.4 Noise Performance
Figure 39 shows the total circuit noise for varying source impedances with the operational amplifier in a unity-
gain configuration (with no feedback resistor network and therefore no additional noise contributions). The
OPA145 and OPA211 are shown with total circuit noise calculated. The op amp itself contributes both a voltage
noise component and a current noise component. The voltage noise is commonly modeled as a time-varying
component of the offset voltage. The current noise is modeled as the time-varying component of the input bias
current and reacts with the source resistance to create a voltage component of noise. Therefore, the lowest noise
op amp for a given application depends on the source impedance. For low source impedance, current noise is
negligible, and voltage noise generally dominates. The OPA145, OPA2145, and OPA4145 family has both low
voltage noise and extremely low current noise because of the FET input of the op amp. As a result, the current
noise contribution of the OPAx145 series is negligible for any practical source impedance, which makes it the
better choice for applications with high source impedance.
The equation in Figure 39 shows the calculation of the total circuit noise, with these parameters:
• e_n = voltage noise
• I_n = current noise
• R_S = source impedance
• k = Boltzmann's constant = 1.38 × 10^–23 J/K
• T = temperature in degrees Kelvin (K)
For more details on calculating noise, see Basic Noise Calculations.
Copyright © 2017, Texas Instruments Incorporated
Submit Documentation Feedback
19
Product Folder Links: OPA145 OPA2145 OPA4145