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LTC1099 Datasheet(PDF) 10 Page - Linear Integrated Systems |
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LTC1099 Datasheet(HTML) 10 Page - Linear Integrated Systems |
10 / 16 page LTC1099 10 ANALOG INTERFACE 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 VIN 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 between REF+ and REF–. Each resistor is nominally 200 Ω giving a total resistance of 3.2k between the reference terminals. When VIN equals REF+, the output code will be all ones. When VIN equals REF–, the output code will be all zeros. Although it is most common to connect REF+ to a 5V reference and REF– to ground, any voltages can be used. The only restrictions are REF+ >REF– and REF+ and REF– 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 as driving VIN. A fast low impedance source is necessary. 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 ground plane with GND (Pin 10) and REF– (Pin 11) tied to it. Bypass capacitors from REF+ (Pin 12) and VCC (Pin 20) 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. |
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