LTC1099

10

The inclusion of a high quality sample-and-hold (S/H)

simplifies the analog interface to the LTC1099. All of the

error terms normally associated with an S/H (hold step,

offset, gain and droop errors) are included in the error

specifications for the A/D. This makes it easy for the

designer since all the error terms need not be taken into

account individually.

S/H Timing

A falling edge on the RD or WR input switches the S/H from

acquire to hold and starts the conversion. The aperture

time is the delay from the falling edge to the actual instant

when the S/H switches to hold. It is typically 110ns.

As soon as a conversion is complete (2.5

µs typ), the S/H

switches back to the sample mode. Even though the

acquisition time is only 240ns, a new conversion cannot

be started for (700ns typ) after a conversion is completed.

Analog Input

The input to the A/D looks like a 60pF capacitor in series

with 550

Ω (Figure 8).

With this high input capacitance care must be taken when

driving the inputs from a source amplifier. When the input

switch closes, an instantaneous capacitive load is applied

to the amplifier output. This acts like an impulse into the

amplifier and if it has poor phase margin the resulting

ringing can cause a considerable loss of accuracy. If the

amplifier is too slow the resulting settling tail will also

cause a loss of accuracy. The amplifier should also have

low open circuit output impedance. The LT1006 is an

1099 F08

550

Ω

TO A/D

60pF

Figure 8. Equivalent Input Circuit

excellent amplifier in this regard. It also works with a single

supply which fits nicely with the LTC1099.

Reference Inputs

Sixteen equal valued resistors are internally connected

Ω

giving a total resistance of 3.2k between the reference

zeros.

must be within the supply rails. As the reference voltage is

reduced the A/D will eventually lose accuracy. Accuracy is

quite good for references down to 1V.

Even though the reference drives a resistive ladder, a lot of

capacitive switching is taking place internally. For this

reason, driving the reference has the same characteristics

The reference has the additional problem of presenting a

DC load to the driving source. This requires the DC as well

as the AC source impedance to be low.

Good Grounding

As with any precise analog system care must be taken to

follow good grounding practices when using the LTC1099.

The most noise free environment is obtained by using a

with short leads are also required to prevent spurious

switching noise from affecting the conversion accuracy.

If a ground plane is not practical, single point grounding

techniques should be used. Ground for the A/D should not

be mixed in with other noisy grounds.