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LTC2757

2757f

APPLICATIONS INFORMATION

swing to ±10V at the output. Compensation is taken from

the output of the LTC6240, allowing the use of a much larger

compensation capacitor than if taken after the gain-of-ﬁve

stage. An LTC2054 auto-zero ampliﬁer senses the voltage

at IOUT1 and drives the non-inverting input of the LTC6240

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 LTC2757 is critical to the performance of

the system, selecting a precision voltage reference also

requires due diligence. The output voltage of the LTC2757

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 appli-

cations: output voltage initial tolerance, output voltage

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%), minimizes the gain error caused by the reference;

however, a calibration sequence that corrects for system

zero- and full-scale error is always recommended.

A reference’s output voltage temperature coefﬁcient affects

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.

As precision DAC applications move to 18-bit performance,

reference output voltage noise may contribute a dominant

share of the system’s noise ﬂoor. This in turn can degrade

system dynamic range and signal-to-noise ratio. Care

should be exercised in selecting a voltage reference with

as low an output noise voltage as practical for the system

resolution desired. Precision voltage references like the

LT1236 produce low output noise in the 0.1Hz to 10Hz

region, well below the 18-bit LSB level in 5V or 10V full-

scale systems. However, as the circuit bandwidths increase,

ﬁltering the output of the reference may be required to

minimize output noise.

Table 6. Partial List of LTC Precision References Recommended

for Use with the LTC2757 with Relevant Speciﬁcations

REFERENCE

INITIAL

TOLERANCE

TEMPERATURE

DRIFT

0.1Hz to 10Hz

NOISE

LT1019A-5,

LT1019A-10

±0.05%

Max

5ppm/°C

Max

12μVP-P

LT1236A-5,

LT1236A-10

±0.05%

Max

5ppm/°C

Max

3μVP-P

LT1460A-5,

LT1460A-10

±0.075%

Max

10ppm/°C

Max

20μVP-P

LT1790A-2.5

±0.05%

Max

10ppm/°C

Max

12μVP-P

LTC6655-2.5

LTC6655-5

±0.025%

Max

2ppm/°C

Max

0.62μVP-P

Grounding

As with any high-resolution converter, clean grounding is

important. A low-impedance analog ground plane is nec-

essary, as are star grounding techniques. Keep the board

layer used for star ground continuous to minimize ground

resistances; that is, use the star-ground concept without

using separate star traces. The IOUT2 pins are of particular

concern; INL will be degraded by the code-dependent

currents carried by the IOUT2F and IOUT2S pins if voltage

drops to ground are allowed to develop. The best strategy

here is to tie the pins to the star ground plane by multiple

vias located directly underneath the part. Alternatively, the

pins may be routed to the star ground point if necessary;

join them together at the part and route a single trace of

no more than 30 squares of 1oz copper.

In the rare case in which neither of these alternatives is

practicable, a force/sense ampliﬁer should be used as a

ground buffer (see the Typical Applications section). Note,

however, that the voltage offset of the ground buffer amp

directly contributes to the effects on accuracy speciﬁed in

Table 4 under ‘VOS1’. The combined effects of the offsets

can be calculated by substituting the total offset from IOUT1

to IOUT2S for VOS1 in the equations.

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