7

LT1220

Input Considerations

Bias current cancellation circuitry is employed on the

inputs of the LT1220 so the input bias current and input

offset current have identical specifications. For this reason,

matching the impedance on the inputs to reduce bias

current errors is not necessary.

Capacitive Loading

The LT1220 is stable with capacitive loads. This is accom-

plished by sensing the load induced output pole and adding

compensation at the amplifier gain node. As the capacitive

load increases, both the bandwidth and phase margin

decrease. There will be peaking in the frequency domain as

shown in the curve of Frequency Response vs Capacitive

Load. The small-signal transient response will have more

overshoot as shown in the photo of the small-signal

response with 1000pF load. The large-signal response with

a 10,000pF load shows the output slew rate being limited

to 4V/

µs by the short-circuit current. The LT1220 can drive

coaxial cable directly, but for best pulse fidelity a resistor of

value equal to the characteristic impedance of the cable

(i.e., 75

Ω) should be placed in series with the output. The

other end of the cable should be terminated with the same

value resistor to ground.

DAC Current-to-Voltage Amplifier

The high gain, low offset voltage, low input bias current,

and fast settling of the LT1220 make it particularly useful

as an I/V converter for current output DACs. A typical

application is shown with a 565A type, 12-bit, 2mA full-

scale output current DAC. The 5k resistor around the

LT1220 is internal to the DAC and gives a 10V full-scale

output voltage. A 5pF capacitor in parallel with the feedback

resistor compensates for the DAC output capacitance and

improves settling. The output of the LT1220 settles to

1/2LSB (1.2mV) in less than 300ns. The accuracy of this

circuit is equal to:

At room temperature the worst-case error is 3mV (1.2LSB).

Typically the error is 1.2mV (1/2LSB). Over the commercial

temperature range the worse-case error is 6mV (2.5LSB).

6 OUT

LT1220 • SS

BIAS 2

BIAS 1

18

NULL

–IN

+IN 3

7

2

4

SI PLIFIED SCHE ATIC