8

LTC2054/LTC2055

sn20545 20545fas

Clock Feedthrough, Input Bias Current

The LTC2054 and LTC2055 use auto-zeroing circuitry to

achieve an almost zero DC offset over temperature,

common mode voltage, and power supply voltage. The

frequency of the clock used for auto-zeroing is typically

1.0kHz. The term clock feedthrough is broadly used to

indicate visibility of this clock frequency in the op amp

output spectrum. There are typically two types of clock

feedthrough in auto zeroed op amps like the LTC2054/

LTC2055.

The first form of clock feedthrough is caused by the

settling of the internal sampling capacitor and is input

referred; that is, it is multiplied by the closed loop gain of

the op amp. This form of clock feedthrough is independent

of the magnitude of the input source resistance or the

magnitude of the gain setting resistors. The LTC2054/

LTC2055 have a residue clock feedthrough of less then

The second form of clock feedthrough is caused by the

small amount of charge injection occurring during the

sampling and holding of the op amp’s input offset voltage.

The current spikes are multiplied by the impedance seen

at the input terminals of the op amp, appearing at the

output multiplied by the closed loop gain of the op amp. To

reduce this form of clock feedthrough, use smaller valued

gain setting resistors and minimize the source resistance

at the input. If the resistance seen at the inputs is less than

10k, this form of clock feedthrough is less than the amount

of residue clock feedthrough from the first form described

above.

Placing a capacitor across the feedback resistor reduces

either form of clock feedthrough by limiting the bandwidth

of the closed loop gain.

Input bias current is defined as the DC current into the

input pins of the op amp. The same current spikes that

APPLICATIO S I FOR ATIO

0.4µV

10 SEC

2054 G16

1µV

1 SEC

2054 G17

LTC2054/LTC2055 DC to 1Hz Noise

LTC2054/LTC2055 DC to 10Hz Noise