LTC1150 datasheet(19/24 Pages) LINER

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LTC2758

2758fb

For more information www.linear.com/LTC2758

applicaTions inForMaTion

Op amp offset contributes mostly to DAC output offset

and gain error, and has minimal effect on INL and DNL.

For example, consider the LTC2758 in unipolar 5V output

range. (Note that for this example, the LSB size is 19µV.)

An op amp offset of 35µV will cause 1.8LSB of output

offset, and 1.8LSB of gain error; but 0.4LSB of INL, and

just 0.1LSB of DNL.

While not directly addressed by the simple equations in

Tables 4 and 5, temperature effects can be handled just as

easily for unipolar and bipolar applications. First, consult

an op amp’s data sheet to find the worst-case VOS and IB

over temperature. Then, plug these numbers in the VOS

and IB equations from Table 5 and calculate the tempera-

ture-induced effects.

For applications where fast settling time is important, Ap-

plication Note 120,

1ppm Settling Time Measurement for

a Monolithic 18-Bit DAC, offers a thorough discussion of

18-bit DAC settling time and op amp selection.

Recommendations

For DC or low-frequency applications, the LTC1150 is the

simplest 18-bit accurate output amplifier. An auto-zero

amp, its exceptionally low offset (10µV max) and offset

drift (0.01µV/°C) make nulling unnecessary. For swings

above 8V, use an LT1010 buffer to boost the load current

capability. The settling of auto-zero amps is a special case;

seeApplicationNote120,

1ppmSettlingTimeMeasurement

for a Monolithic 18-Bit DAC, Appendix E, for details.

TheLT1012andLT1001aregoodintermediateoutput-amp

solutionsthatachievemoderatespeedandgoodaccuracy.

They are also excellent choices for the reference inverting

amplifier in fixed-reference applications.

For high speed applications, the LTC1468 settles in 2.1µs.

Note that the 75µV max offset will degrade the INL at the

DAC output by up to 0.9LSB. For high-speed applications

demanding higher precision, the amplifier offset can be

nulled with a digital potentiometer.

TheTypicalApplicationonthelastpageshowsacomposite

output amplifier that achieves fast settling (8µs) and very

low offset (3µV max) without offset nulling. This circuit

offers high open-loop gain (1000V/mV min), low input

bias current (0.15nA max), fast slew rate (25V/µs min),

and a high gain-bandwidth product (30MHz typ). The high

speed path consists of an LTC6240HV, which is an 18MHz

ultralow bias current amplifier, followed by an LT1360, a

50MHz fast-slewing amplifier which provides additional

gain and the ability to swing to ±10V at the output. Com-

pensation is taken from the output of the LTC6240HV,

allowing the use of a much larger compensation capacitor

than if taken after the gain-of-five stage. An LTC2054HV

auto-zero amplifier senses the voltage at IOUT1 and drives

the non-inverting input of the LTC6240HV to eliminate the

offset of the high speed path. The 100:1 attenuator and

input filter reduce the low frequency noise in this stage

while maintaining low DC offset.

Precision Voltage Reference Considerations

Much in the same way selecting an operational amplifier

for use with the LTC2758 is critical to the performance of

the system, selecting a precision voltage reference also

requires due diligence. The output voltage of the LTC2758

is directly affected by the voltage reference; thus, any

voltage reference error will appear as a DAC output volt-

age error.

There are three primary error sources to consider

when selecting a precision voltage reference for 18-bit

applications:outputvoltageinitialtolerance,outputvoltage

temperature coefficient and output voltage noise.

Initial reference output voltage tolerance, if uncorrected,

generates a full-scale error term. Choosing a reference

with low output voltage initial tolerance, like the LT1236

(±0.05%),minimizesthegainerrorcausedbythereference;

however, a calibration sequence that corrects for system

zero- and full-scale error is always recommended.

Areference’soutputvoltagetemperaturecoefficientaffects

not only the full-scale error, but can also affect the circuit’s

INL and DNL performance. If a reference is chosen with

a loose output voltage temperature coefficient, then the

DAC output voltage along its transfer characteristic will

be very dependent on ambient conditions. Minimizing

the error due to reference temperature coefficient can be

achieved by choosing a precision reference with a low

output voltage temperature coefficient and/or tightly con-

trolling the ambient temperature of the circuit to minimize

temperature gradients.

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