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HIGH IMPEDANCE MODE

LINEARITY ERRORvsSOURCEIMPEDANCE

ExternalSourceImpedance(k

Ω)

10

9

8

7

6

5

4

3

2

1

0

0

1

2

3

4

5

6

7

8

9 10 11 12

14 15

13

T

Acquisition Time=5µs

ILE

DLE

DRIVING THE ADS8513 ANALOG INPUT

ADS8513

SLAS486 – JUNE 2007

Likewise, three of the input ranges involve the connection at R2IN being driven below GND. This input has a

reverse biased ESD protection diode connection to ground. If R2IN is taken below GND – 0.3V, this diode will be

forward-biased and will clamp the negative input at –0.4V to –0.7V, depending on the temperature. Since the

negative full-scale value of these input ranges exceed –0.4V, they are not recommended.

Note that Table 4 assumes that the voltage at the REF pin is +2.5V. This is true if the internal reference is being

used or if the external reference is +2.5V. Other reference voltages will change the values inTable 4.

When R1IN, R2IN, and R3IN are connected to the analog input, the input range of the ADS8513 is 0.3125V to

2.8125V and the input impedance is greater than 10M

Ω. This input range can be used to connect the ADS8513

directly to a wide variety of sensors. Figure 12 shows the impedance of the sensor versus the change in ILE and

DLE of the ADS8513. The performance of the ADS8513 can be improved for higher sensor impedance by

allowing more time for acquisition. For example, 10 s of acquisition time will approximately double sensor

impedance for the same ILE/DLE performance.

The input impedance and capacitance of the ADS8513 are very stable with temperature. Assuming that this is

true of the sensor as well, the graph shown in Figure 12 will vary less than a few percent over the ensured

temperature range of the ADS8513. If the sensor impedance varies significantly with temperature, the worst-case

impedance should be used.

Figure 12. Linearity Error vs Source Impedance in the High Impedance Mode (R1IN = R2IN = R3IN = VIN)

In general, any reasonably fast, high-quality operational or instrumentation amplifier can be used to drive the

ADS8513 input. When the converter enters the acquisition mode, there is some charge injection from the

converter input to the amplifier output. This can result in inadequate settling time with slower amplifiers. Be very

careful with single-supply amplifiers, particularly if their output will be required to swing very close to the supply

rails.

In addition, be careful in regards to the amplifier linearity. The outputs of single-supply and rail-to-rail amplifiers

can saturate as they approach the supply rails. Rather than the amplifier transfer function being a straight line,

the curve can become severely ‘S’ shaped. Also, watch for the point where the amplifier switches from sourcing

current to sinking current. For some amplifiers, the transfer function can be noticeably discontinuous at this point,

causing a significant change in the output voltage for a much smaller change on the input.

Texas Instruments manufactures a wide variety of operational and instrumentation amplifiers that can be used to

drive the input of the ADS8513. These include the OPA627, OPA132, and INA110.

14

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