Datasheet LT1220 (Analog Devices) - 7

LT1220
U U W U APPLICATIONS INFORMATION Input Considerations
(i.e., 75Ω) should be placed in series with the output. The other end of the cable should be terminated with the same Bias current cancellation circuitry is employed on the value resistor to ground. inputs of the LT1220 so the input bias current and input offset current have identical specifications. For this reason,
DAC Current-to-Voltage Amplifier
matching the impedance on the inputs to reduce bias current errors is not necessary. The high gain, low offset voltage, low input bias current, and fast settling of the LT1220 make it particularly useful
Capacitive Loading
as an I/V converter for current output DACs. A typical application is shown with a 565A type, 12-bit, 2mA full- The LT1220 is stable with capacitive loads. This is accom- scale output current DAC. The 5k resistor around the plished by sensing the load induced output pole and adding LT1220 is internal to the DAC and gives a 10V full-scale compensation at the amplifier gain node. As the capacitive output voltage. A 5pF capacitor in parallel with the feedback load increases, both the bandwidth and phase margin resistor compensates for the DAC output capacitance and decrease. There will be peaking in the frequency domain as improves settling. The output of the LT1220 settles to shown in the curve of Frequency Response vs Capacitive 1/2LSB (1.2mV) in less than 300ns. The accuracy of this Load. The small-signal transient response will have more circuit is equal to: overshoot as shown in the photo of the small-signal response with 1000pF load. The large-signal response with VERROR = VOS + (IOS • 5kΩ) + (VOUT/AVOL) a 10,000pF load shows the output slew rate being limited At room temperature the worst-case error is 3mV (1.2LSB). to 4V/µs by the short-circuit current. The LT1220 can drive Typically the error is 1.2mV (1/2LSB). Over the commercial coaxial cable directly, but for best pulse fidelity a resistor of temperature range the worse-case error is 6mV (2.5LSB). value equal to the characteristic impedance of the cable
W W SI PLIFIED SCHE ATIC
V+ 7 NULL 1 8 BIAS 1 BIAS 2 6 OUT +IN 3 2 –IN V – 4 LT1220 • SS 7