LTC1250

7

1250fb

LTC1250

47k

0.01

1250 F04

–

+

1250 F03

LTC1250

LTC1250 and on the PC board, play an increasing role.

Low value resistors (below 5k) may not require a capaci-

tor at all.

Input Bias Current

The inputs of the LTC1250, like all zero-drift op amps,

draw only small switching spikes of AC bias current; DC

leakage current is negligible except at very high tempera-

tures. The large front-end transistors cause switching

spikes 3 to 4 times greater than standard zero-drift op

amps: the ±50pA bias current spec is still many times

better than most bipolar parts. The spikes don’t match

from one input pin to the other, and are sometimes (but

not always) of opposite polarity. As a result, matching the

impedances at the inputs (Figure 3) will not cancel the

bias current, and may cause additional errors. Don’t do it.

when the LTC1250’s output is heavily loaded, the chip

may dissipate substantial power, raising the temperature

of the package and aggravating thermocouples at the

inputs. Although the LTC1250 will maintain its specified

accuracy under these conditions, care must be taken in

the layout to prevent or compensate circuit errors. Be

especially careful of air currents when measuring low

frequency noise; nearby moving objects (like people) can

create very large noise peaks with an unshielded circuit

board. For more detailed explanations and advice on how

to avoid these errors, see the LTC1051/LTC1053 data

sheet.

Sampling Behavior

The LTC1250’s zero-drift nulling loop samples the input at

≈ 5kHz, allowing it to process signals below 2kHz with no

aliasing. Signals above this frequency may show aliasing

behavior, although wideband internal circuitry generally

keeps errors to a minimum. The output of the LTC1250

will have small spikes at the clock frequency and its

harmonics; these will vary in amplitude with different

feedback configurations. Low frequency or band-limited

systems should not be affected, but systems with higher

bandwidth (oversampling A/Ds, for example) may need to

filter out these clock artifacts. Output spikes can be

minimized with a large feedback capacitor, but this will

adversely affect noise performance (see Input Capaci-

tance and Compensation on the previous page). Applica-

tions which require spike-free output in addition to mini-

mum noise will need a low-pass filter after the LTC1250;

a simple RC will usually do the job (Figure 4). The

LTC1051/LTC1053 data sheet includes more information

about zero-drift amplifier sampling behavior.

Figure 3. Extra Resistor Will

Not Cancel Bias Current Errors

S

APPLICATI

I FOR ATIO

Output Drive

The LTC1250 includes an enhanced output stage which

provides nearly symmetrical output source/sink currents.

This output is capable of swinging a minimum of ±4V into

a 1k load with ±5V supplies, and can sink orsource>20mA

LTC1250 will swing to within millivolts of either rail. In

single supply applications, it will typically swing 4.3V into

a 1k load with a 5V supply.

Minimizing External Errors

The input noise, offset voltage, and bias current specs for

the LTC1250 are all well below the levels of circuit board

parasitics. Thermocouples between the copper pins of the

LTC1250 and the tin/lead solder used to connect them can

overwhelm the offset voltage of the LTC1250, especially

if a soldering iron has been around recently. Note also that

Figure 4. RC Output Pole Limits Bandwidth to 330Hz