7

LTC1051/LTC1053

10513fa

As the ambient temperature rises, the leakage current of

the input protection devices increases, while the charge

injection component of the bias current, for all practical

purposes, stays constant. At elevated temperatures (above

85°C) the leakage current dominates and the bias current

of both inputs assumes the same sign.

The charge injection at the op amp input pins will cause

small output spikes. This phenomenon is often referred to

as “clock feedthrough” and can be easily observed when

the closed-loop gain exceeds 10V/V (Figure 2). The mag-

nitude of the clock feedthrough is temperature indepen-

dent but it increases when the closed-loop gain goes up,

when the source resistance increases and when the gain

setting resistors increase (Figure 2a, 2b). It is important to

note that the output small spikes are centered at 0V level

and do not add to the output offset error budget. For

of Figure 2c is:

A 10pA bias current will yield an output of 1mV ±100µV.

The output clock feedthrough can be attenuated by lower-

ing the value of the gain setting resistors, i.e. R2 = 10k,

R1 = 100Ω, instead of 100k and 1k (Figure 2).

Clock feedthrough can also be attenuated by adding a

capacitor across the feedback resistor to limit the circuit

bandwidth below the internal sampling frequency

(Figure 3).

Input Capacitance

The input capacitance of the LTC1051/LTC1053 op amps

is approximately 12pF. When the LTC1051/LTC1053 op

amps are used with feedback factors approaching unity,

the feedback resistor value should not exceed 7k for

industrial temperature range and 5k for military tempera-

ture range. If a higher feedback resistor value is required,

a feedback capacitor of 20pF should be placed across the

feedback resistor. Note that the most common circuits

with feedback factors approaching unity are unity gain

followers and instrumentation amplifier front ends.

(See Figure 4.)

Figure 2. Clock Feedthrough

Figure 3. Adding a Feedback Capacitor to

Eliminate Clock Feedthrough

–

+

1/2

LTC1051

1051/53 F02

(c)

100µs/DIV

(b)

100µs/DIV

(a)

R1

1k

R2

100k

20mV/DIV

20mV/DIV

20mV/DIV

20mV/DIV

Figure 4. Operating the LTC1051

with Feedback Factors Approaching Unity

–

+

1/2

LTC1051

1051/53 F04

R1

R2 < 7k, IF R1 > >R2

1

2

3

–

+

1/2

LTC1051

1051/53 F03

R1

1k

R2

100k

C

1000pF

1

2

3

100µs/DIV

APPLICATIO S I FOR ATIO