Datasheet LTC1099 (Analog Devices) - 10
| Manufacturer | Analog Devices |
| Description | High Speed 8-Bit A/D Converter with Built-In Sample-and-Hold |
| Pages / Page | 16 / 10 — ANALOG INTERFACE. Reference Inputs. S/H Timing. Analog Input. Good … |
| File Format / Size | PDF / 204 Kb |
| Document Language | English |
ANALOG INTERFACE. Reference Inputs. S/H Timing. Analog Input. Good Grounding. Figure 8. Equivalent Input Circuit
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LTC1099
U U ANALOG INTERFACE
The inclusion of a high quality sample-and-hold (S/H) excellent amplifier in this regard. It also works with a single simplifies the analog interface to the LTC1099. All of the supply which fits nicely with the LTC1099. error terms normally associated with an S/H (hold step, offset, gain and droop errors) are included in the error
Reference Inputs
specifications for the A/D. This makes it easy for the Sixteen equal valued resistors are internally connected designer since all the error terms need not be taken into between REF+ and REF–. Each resistor is nominally 200Ω account individually. giving a total resistance of 3.2k between the reference terminals. When V
S/H Timing
IN equals REF +, the output code will be all ones. When VIN equals REF–, the output code will be all A falling edge on the RD or WR input switches the S/H from zeros. acquire to hold and starts the conversion. The aperture Although it is most common to connect REF+ to a 5V time is the delay from the falling edge to the actual instant reference and REF– to ground, any voltages can be used. when the S/H switches to hold. It is typically 110ns. The only restrictions are REF+ >REF– and REF+ and REF– As soon as a conversion is complete (2.5µs typ), the S/H must be within the supply rails. As the reference voltage is switches back to the sample mode. Even though the reduced the A/D will eventually lose accuracy. Accuracy is acquisition time is only 240ns, a new conversion cannot quite good for references down to 1V. be started for (700ns typ) after a conversion is completed. Even though the reference drives a resistive ladder, a lot of
Analog Input
capacitive switching is taking place internally. For this reason, driving the reference has the same characteristics The input to the A/D looks like a 60pF capacitor in series as driving VIN. A fast low impedance source is necessary. with 550Ω (Figure 8). The reference has the additional problem of presenting a 550Ω DC load to the driving source. This requires the DC as well VIN TO A/D as the AC source impedance to be low. 60pF 1099 F08
Good Grounding Figure 8. Equivalent Input Circuit
As with any precise analog system care must be taken to follow good grounding practices when using the LTC1099. With this high input capacitance care must be taken when The most noise free environment is obtained by using a driving the inputs from a source amplifier. When the input ground plane with GND (Pin 10) and REF– (Pin 11) tied to switch closes, an instantaneous capacitive load is applied it. Bypass capacitors from REF+ (Pin 12) and V to the amplifier output. This acts like an impulse into the CC (Pin 20) with short leads are also required to prevent spurious amplifier and if it has poor phase margin the resulting switching noise from affecting the conversion accuracy. ringing can cause a considerable loss of accuracy. If the amplifier is too slow the resulting settling tail will also If a ground plane is not practical, single point grounding cause a loss of accuracy. The amplifier should also have techniques should be used. Ground for the A/D should not low open circuit output impedance. The LT1006 is an be mixed in with other noisy grounds. 10
U U ANALOG INTERFACE
The inclusion of a high quality sample-and-hold (S/H) excellent amplifier in this regard. It also works with a single simplifies the analog interface to the LTC1099. All of the supply which fits nicely with the LTC1099. error terms normally associated with an S/H (hold step, offset, gain and droop errors) are included in the error
Reference Inputs
specifications for the A/D. This makes it easy for the Sixteen equal valued resistors are internally connected designer since all the error terms need not be taken into between REF+ and REF–. Each resistor is nominally 200Ω account individually. giving a total resistance of 3.2k between the reference terminals. When V
S/H Timing
IN equals REF +, the output code will be all ones. When VIN equals REF–, the output code will be all A falling edge on the RD or WR input switches the S/H from zeros. acquire to hold and starts the conversion. The aperture Although it is most common to connect REF+ to a 5V time is the delay from the falling edge to the actual instant reference and REF– to ground, any voltages can be used. when the S/H switches to hold. It is typically 110ns. The only restrictions are REF+ >REF– and REF+ and REF– As soon as a conversion is complete (2.5µs typ), the S/H must be within the supply rails. As the reference voltage is switches back to the sample mode. Even though the reduced the A/D will eventually lose accuracy. Accuracy is acquisition time is only 240ns, a new conversion cannot quite good for references down to 1V. be started for (700ns typ) after a conversion is completed. Even though the reference drives a resistive ladder, a lot of
Analog Input
capacitive switching is taking place internally. For this reason, driving the reference has the same characteristics The input to the A/D looks like a 60pF capacitor in series as driving VIN. A fast low impedance source is necessary. with 550Ω (Figure 8). The reference has the additional problem of presenting a 550Ω DC load to the driving source. This requires the DC as well VIN TO A/D as the AC source impedance to be low. 60pF 1099 F08
Good Grounding Figure 8. Equivalent Input Circuit
As with any precise analog system care must be taken to follow good grounding practices when using the LTC1099. With this high input capacitance care must be taken when The most noise free environment is obtained by using a driving the inputs from a source amplifier. When the input ground plane with GND (Pin 10) and REF– (Pin 11) tied to switch closes, an instantaneous capacitive load is applied it. Bypass capacitors from REF+ (Pin 12) and V to the amplifier output. This acts like an impulse into the CC (Pin 20) with short leads are also required to prevent spurious amplifier and if it has poor phase margin the resulting switching noise from affecting the conversion accuracy. ringing can cause a considerable loss of accuracy. If the amplifier is too slow the resulting settling tail will also If a ground plane is not practical, single point grounding cause a loss of accuracy. The amplifier should also have techniques should be used. Ground for the A/D should not low open circuit output impedance. The LT1006 is an be mixed in with other noisy grounds. 10
