AD7575JPZ Datasheet AD7575AQ, AD7575BQ, AD7575JNZ | P10-P12

AD7575

–9–

REV. B

UNIPOLAR OPERATION

The basic operation for the AD7575 is in the unipolar single

supply mode. Figure 15 shows the circuit connections to achieve

this, while the nominal transfer characteristic for unipolar opera-

tion is given in Figure 16. Since the offset and full-scale errors

on the AD7575 are very small, in many cases it will not be nec-

essary to adjust out these errors. If calibration is required, the

procedure is as follows:

Offset Adjust

Offset error adjustment in single-supply systems is easily achiev-

able by means of the offset null facility of an op amp when used

as a voltage follower for the analog input signal, AIN. The op

amp chosen should be able to operate from a single supply and

allow a common-mode input voltage range that includes 0 V

(e.g., TLC271). To adjust for zero offset, the input signal

source is set to +4.8 mV (i.e., 1/2 LSB) while the op amp offset

is varied until the ADC output code flickers between 000 . . . 00

and 000 . . . 01.

Full-Scale Adjust

The full scale or gain adjustment is made by forcing the analog

input AIN to +2.445 V (i.e., Full-Scale Voltage –3/2 LSB). The

magnitude of the reference voltage is then adjusted until the

ADC output code flickers between 111 . . . 10 and 111. . . 11.

BIPOLAR OPERATION

The circuit of Figure 17 shows how the AD7575 can be config-

ured for bipolar operation. The output code provided by the

AD7575 is offset binary. The analog input voltage range is

±5 V, although the voltage appearing at the AIN pin of the

AD7575 is in the range 0 V to +2.46 V. Figure 18 shows the

transfer function for bipolar operation. The LSB size is now

39.06 mV. Calibration of the bipolar operation is outlined be-

low. Once again, because the errors are small, it may not be

necessary to adjust them. To maintain specified performance

without the calibration, all resistors should be 0.1% tolerance

with R4 and R5 replaced by one 3.3 kΩ resistor and R2 and R3

replaced by one 2.5 kΩ resistor.

Offset Adjust

Offset error adjustment is achieved by applying an analog input

voltage of –4.9805 V (–FS +1/2 LSB). Resistor R3 is then

adjusted until the output code flickers between 000 . . . 00 and

000 . . . 01.

Full-Scale Adjust

Full-scale or gain adjustment is made by applying an analog

input voltage of +4.9414 V (+FS –3/2 LSB). Resistor R4 is then

adjusted until the output code flickers between 111 . . . 10 and

111. . . 11.

47mF 0.1mF

+5V

+2.46V

MAX

47mF 0.1mF

–

AD589

3.3kV

+5V

+1.23V

CONTROL

INPUTS

+5V

AIN

REF

AGND

CLK

BUSY

DB7–DB0

DATA OUT

AD7575

DGND

+5V

CLK

100kV, 1%

CLK

100pF, 2%

Figure 15. Unipolar Configuration

OUTPUT

CODE

FULL SCALE

TRANSITION

11111111

11111110

11111101

00000011

00000010

00000001

00000000

FS = 2V

REF

1LSB =

256

1LSB 3LSBs

2LSBs

FS –1LSB

AIN, INPUT VOLTAGE (IN TERMS OF LSBs)

Figure 16. Nominal Transfer Characteristic for

Unipolar Operation

47mF 0.1mF

+5V

47mF 0.1mF

–

AD589

+5V

AIN

REF

CLK

BUSY

DB7–DB0

DATA OUT

AD7575

DGND

+5V

CLK

100kV, 1%

CLK

100pF, 2%

AGND

2.2kV

500V

INPUT

VOLTAGE

500V

3kV

+5V

2.5kV

TLC271

3.3kV

10kV

Figure 17. Bipolar Configuration

–1/2LSB

+1/2LSB

–FS

+FS –1LSB

AIN

FS = 5V

1LSB =

256

OUTPUT

CODE

111...111

111...110

100...010

100...001

100...000

011...111

011...110

000...001

000...000

Figure 18. Nominal Transfer Characteristic for

Bipolar Operation

AD7575

–10–

REV. B

APPLICATION HINTS

1. NOISE: Both the input signal lead to AIN and the signal

return lead from AGND should be kept as short as possible to

minimize input-noise coupling. In applications where this is

not possible, either a shielded cable or a twisted pair transmis-

sion line between source and ADC is recommended. Also,

since any potential difference in grounds between the signal

source and ADC appears as an error voltage in series with the

input signal, attention should be paid to reducing the ground

circuit impedance as much as possible. In general, the source

resistance should be kept below 2 kΩ. Larger values of source

resistance can cause undesired system noise pickup.

2. PROPER LAYOUT: Layout for a printed circuit board

should ensure that digital and analog lines are kept separated

as much as possible. In particular, care should be taken not to

run any digital track alongside an analog signal track. Both the

analog input and the reference input should be screened by

AGND. A single point analog ground separate from the logic

system ground, should be established at or near the AD7575.

This single point analog ground subsystem should be con-

nected to the digital system ground by a single-track connec-

tion only. Any reference bypass capacitors, analog input filter

capacitors or input signal shielding should be returned to the

analog ground point.

AD7575 WITH AD589 REFERENCE

The AD7575 8-bit A/D converter features a total unadjusted

error specification over its entire operating temperature range.

This total unadjusted error includes all errors in the A/D con-

verter—offset, full scale and linearity. The one feature not pro-

vided on the AD7575 is a voltage reference. This section

discusses the use of the AD589 bandgap reference with the

AD7575, and gives the combined reference and ADC error

budget over the full operating temperature range. This allows

the user to compare the combined AD589/AD7575 errors to

ADCs whose specifications include on-chip references.

Two distinct application areas exist. The first is where the refer-

ence voltage and the analog input voltage are derived from the

same source. In other words, if the reference voltage varies, the

analog input voltage range varies by a ratioed amount. In this

case, the user is not worried about the absolute value of the

reference voltage. The second case is where changes in the refer-

ence voltage are not matched by changes in the analog input

voltage range. Here, the absolute value of the reference voltage,

and its drift over temperature, are of prime importance. Both

applications are discussed below.

If the analog input range varies with the reference voltage, the

part is said to be operating ratiometrically. This is representative

of many applications. If the reference is on-chip, and the user

does not have access to it, it is not possible to get ratiometric

operation. Since the AD7575 uses an external reference, it can

be used in ratiometric applications. However, because the part is

specified with a reference of +1.23 V ± 5%, then the voltage

range for ratiometric operation is limited.

The error analysis over temperature of ratiometric applications

is different from nonratiometric ones. Since the reference and

analog input voltage range are ratioed to each other, tempera-

ture variations in the reference are matched by variations in the

analog input range. Therefore, the AD589 contributes no addi-

tional errors over temperature to the system errors, and the

combined total unadjusted error specification for the AD589

and AD7575 is as per the total unadjusted error specification in

this data sheet.

With nonratiometric applications, however, the analog input

range stays the same if the reference varies and a full-scale error

is introduced. The amount by which the reference varies deter-

mines the amount of error introduced. The AD589 is graded on

temperature coefficient; therefore, selection of different grades

allows the user to tailor the amount of error introduced to suit

the system requirements. The reference voltage from the AD589

can lie between 1.2 V and 1.25 V. This reference voltage can be

adjusted for the desired full-scale voltage range using the circuit

outlined in Figure 19. For example, if an analog input voltage

range of 0 V to +2.46 V is required, the reference should be

adjusted to +1.23 V. Once the reference is adjusted to the de-

sired value at 25°C, the total error is as per the total unadjusted

error specification on the AD7575 specification pages. (To

reduce this still further, offset and full-scale errors of the

AD7575 can be adjusted out using the calibration procedure

outlined in this data sheet.)

TLC271*

+5V

–

6.8kV

+5V

10kV*1kV*

10kV*

AD589

*ONLY REQUIRED IF IT IS NECESSARY TO ADJUST

THE ABSOLUTE VALUE OF REFERENCE VOLTAGE.

Figure 19. Reference Adjust Circuit

However, it is as the temperature varies from 25°C that the

AD589 starts to introduce errors. The typical temperature char-

acteristics of the AD589 are shown in Figure 20. The tempera-

ture coefficients (TCs) represent the slopes of the diagonals of

the error band from +25°C to T

MIN

and +25°C to T

MAX

. The

AD589 TC is specified in ppm/°C max and is offered in four

different grades.

AD7575

–11–

REV. B

Taking the 25°C measurement as the starting point, the

full-scale error introduced is always in the negative direction

whether the temperature goes to T

MIN

or T

MAX

. This can be

seen from the AD589 temperature characteristic shown in Fig-

ure 20. If the reference voltage is adjusted for 1.23 V at 45°C

(for the 0°C to +70°C range) and 75°C (for the –55°C to

+125°C range) the magnitude of the error introduced is reduced

since it is distributed in both the positive and negative direc-

tions. Alternatively, this can be achieved not by adjusting at

these temperatures, which would be impractical, but by adjust-

ing the reference to 1.231 V instead of 1.23 V (for the extended

temperature range) at 25°C. This has the required effect of

distributing the plot of Figure 20 more evenly about the desired

value.

An additional error source is the mismatch between the tem-

perature coefficients (TCs) of the 10 kΩ and 1 kΩ resistors in

the feedback loop of the TLC271. If these resistors have

±50 ppm/°C absolute TCs, the worst case difference in drift be-

tween both resistors is 100 ppm/°C. From +25°C to +125°C, this

introduces a worst case shift of 1.22 mV, which results in an addi-

tional full-scale error of 0.25 LSB. If ±25 ppm/°C

resistors are

used, then the worst case error is 0.13 LSB. Over the 0°C to

+70°C range, the ±50 ppm/°C resistors introduce an additional

full-scale error of 0.11 LSB. All these errors are worst case and

assume that the resistance values drift in opposite directions. In

practice, resistors of the same type, and from the same manufac-

turer, would drift in the same direction and hence the above

error would be considerably reduced. An additional error source

is the offset drift of the TLC271. This is significant only over

the –55°C to +125°C range and, even in this case, it contrib-

utes <0.1 LSB worth of full-scale error.

The error outlined in the right-hand column of Table I is a total

unadjusted error specification, excluding resistor and offset drift

(the effect of these can be controlled by the user). It consists of

errors from two error sources: a ±l LSB contribution from the

AD7575 (including full-scale, offset and relative accuracy er-

rors), and the remainder is a full-scale error introduced by the

AD589. It is important to note that the variation of the AD589

voltage only introduces a full-scale error; the relative accuracy

(or endpoint nonlinearity) of the system, with a top grade

AD7575, is still ±1/2 LSB (i.e., 8-bits accurate).

TEMPERATURE –

1.2370

1.2365

1.2345

–50 125–25

OUTPUT VOLTAGE – V

0 255075100

1.2360

1.2355

1.2350

Figure 20. Typical AD589 Temperature Characteristics

The effect the TC has on the system error is that it introduces a

full-scale error in the ADC. This, in turn, affects the total unad-

justed error specification. For example, using the AD589KH

with a 50 ppm/°C max TC the change in reference voltage from

25°C to 70°C will be from 1.23 V to 1.22724 V, a change of –

2.76 mV. This results in a change in the full-scale range of the

ADC of –5.52 mV, since the full-scale range on the AD7575 is

2 V

REF

. Because the LSB size for the AD7575 is 9.61 mV, the

AD589 introduces an additional full-scale error of –0.57 LSBs

on top of the existing full-scale error specification for the ADC.

Since the total unadjusted error specification for the ADC

includes the full-scale error, there is also a corresponding in-

crease in the total unadjusted error of –0.57 LSBs. The change

in reference voltage at 0°C is –1.5 mV, resulting in a full-scale

change of –3 mV or –0.31 LSBs worth of full-scale error. Table I

shows the amount of additional total unadjusted error, which is

introduced by the temperature variation of the AD589, for

different grades and for different temperature ranges. This table

applies only to nonratiometric applications, because the tem-

perature variation of the reference does not affect the system

error in ratiometric applications as outlined earlier. It shows the

amount of error introduced over T

MIN

to T

MAX

for a system in

which the reference has been adjusted to the desired value at

25°C. The final or right-most column of the table gives the total

combined error for the AD589 and the top grade AD7575.

Table I. AD589/AD7575 Error over Temperature (Nonratiometric Applications)

Full-Scale Error Introduced Combined Worst Case

AD589 Temperature by AD589 @ T

MAX

AD589/AD7575

Grade Range (Worst Case) T.U.E. @ T

MAX

AD589JH 0°C to +70°C –1.15 LSB –2.15 LSB

AD589KH 0°C to +70°C –0.57 LSB –1.57 LSB

AD589LH 0°C to +70°C –0.29 LSB –1.29 LSB

AD589MH 0°C to +70°C –0.115 LSB –1.115 LSB

AD589SH –55°C to +125°C –2.56 LSB –3.56 LSB

AD589TH –55°C to +125°C –1.28 LSB –2.28 LSB

AD589UH –55°C to +125°C –0.64 LSB –1.64 LSB

*Excluding resistor and offset drift.

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