AD7910ARMZ Datasheet AD7920AKSZ-500RL7, AD7920BRMZ, AD7910AKSZ-500RL7 | P13-P15

AD7910/AD7920

Rev. C | Page 13 of 24

CIRCUIT INFORMATION

The AD7910/AD7920 are fast, micropower, 10-bit/12-bit,

single-supply A/D converters, respectively. The parts can be

operated from a 2.35 V to 5.25 V supply. When operated from

either a 5 V supply or a 3 V supply, the AD7910/AD7920 are

capable of throughput rates of 250 kSPS when provided with a

5 MHz clock.

The AD7910/AD7920 provide the user with an on-chip track-

and-hold, A/D converter, and a serial interface housed in a tiny

6-lead SC70 package or 8-lead MSOP package, which offers the

user considerable space saving advantages over alternative

solutions.

The serial clock input accesses data from the part but also

provides the clock source for the successive approximation A/D

converter. The analog input range is 0 V to V

. An external

reference is not required for the ADC and there is no reference

on-chip. The reference for the AD7910/AD7920 is derived from

the power supply and thus gives the widest dynamic input

range.

The AD7910/AD7920 also feature a power-down option to

allow power saving between conversions. The power-down

feature is implemented across the standard serial interface, as

described in the

Modes of Operation section.

AD7910/AD7920

Rev. C | Page 14 of 24

CONVERTER OPERATION

The AD7910/AD7920 are successive approximation analog-to-

digital converters based around a charge redistribution DAC.

Figure 14 and Figure 15 show simplified schematics of the

ADC.

Figure 14 shows the ADC during its acquisition phase.

When SW2 is closed and SW1 is in Position A, the comparator

is held in a balanced condition, and the sampling capacitor

acquires the signal on V

CHARGE

REDISTRIBUTION

DAC

CONTROL

LOGIC

COMPARATOR

SW2

SAMPLING

CAPACITOR

ACQUISITION

PHASE

SW1

AGND

02976-014

Figure 14. ADC Acquisition Phase

When the ADC starts a conversion (see Figure 15), SW2 opens

and SW1 moves to Position B, causing the comparator to become

unbalanced. The control logic and charge redistribution DAC are

used to add and subtract fixed amounts of charge from the

sampling capacitor to bring the comparator back into a balanced

condition. When the comparator is rebalanced, the conversion is

complete. The control logic generates the ADC output code.

Figure 16 shows the ADC transfer function.

CHARGE

REDISTRIBUTION

DAC

CONTROL

LOGIC

COMPARATOR

SW2

SAMPLING

CAPACITOR

CONVERSION

PHASE

SW1

AGND

02976-015

Figure 15. ADC Conversion Phase

ADC TRANSFER FUNCTION

The output coding of the AD7910/AD7920 is straight binary.

The designed code transitions occur at the successive integer

LSB values, that is, 1 LSB, 2 LSBs, and so on. The LSB size is

/4096 for the AD7920 and V

/1024 for the AD7910. The

ideal transfer characteristic for the AD7910/AD7920 is shown

Figure 16.

000...000

ADC CODE

ANALOG INPUT

111...111

000...001

000...010

111...110

111...000

011...111

1LSB

– 1LSB

1LSB = V

/1024 (AD7910)

1LSB = V

/4096 (AD7920)

02976-016

Figure 16. Transfer Characteristic

TYPICAL CONNECTION DIAGRAM

Figure 17 shows a typical connection diagram for the AD7910/

AD7920. V

REF

is taken internally from V

and, as such, V

should be well decoupled. This provides an analog input range of

0 V to V

. The conversion result is output in a 16-bit word with

four leading zeros followed by the MSB of the 12-bit or 10-bit

result. Two trailing zeros follow the 10-bit result from the

AD7910.

Alternatively, because the supply current required by the

AD7910/AD7920 is so low, a precision reference can be used as

the supply source to the AD7910/AD7920. An REF19x voltage

reference (REF195 for 5 V or REF193 for 3 V) can be used to

supply the required voltage to the ADC (see

Figure 17). This

configuration is especially useful if the power supply is quite

noisy or if the system supply voltages are at a value other than

5 V or 3 V (for example, 15 V). The REF19x outputs a steady

voltage to the AD7910/AD7920. If the low dropout REF193 is

used, the current it needs to supply to the AD7910/AD7920 is

typically 1.2 mA. When the ADC is converting at a rate of

250 kSPS, the REF193 needs to supply a maximum of 1.4 mA to

the AD7910/AD7920. The load regulation of the REF193 is

typically 10 ppm/mA (REF193, V

= 5 V), which results in an

error of 14 ppm (42 μV) for the 1.4 mA drawn from it. This

corresponds to a 0.057 LSB error for the AD7920 with V

3 V from the REF193 and a 0.014 LSB error for the AD7910.

For applications where power consumption is of concern, the

power-down mode of the ADC and the sleep mode of the

REF19x reference should be used to improve power

performance. See the

Modes of Operation section.

AD7910/AD7920

Rev. C | Page 15 of 24

AD7910/

AD7920

SERIAL

INTERFACE

0V TO V

INPUT

GND

SCLK

SDATA

0.1

F10

TANT

0.1

680nF

SUPPLY

1.2mA

REF193

02976-017

Figure 17. REF193 as Power Supply

Table 6 provides typical performance data with various

references used as a V

source for a 100 kHz input tone at

room temperature, under the same setup conditions.

Table 6. AD7920 Typical Performance for Various Voltage

References IC

Reference Tied to V

AD7920 SNR Performance (dB)

AD780 @ 3 V 72.65

REF193 72.35

AD780 @ 2.5 V 72.5

REF192 72.2

REF43 72.6

ANALOG INPUT

Figure 18 shows an equivalent circuit of the analog input

structure of the AD7910/AD7920. The two diodes, D1 and D2,

provide ESD protection for the analog input. Care must be

taken to ensure that the analog input signal never exceeds the

supply rails by more than 300 mV. This causes these diodes to

become forward biased and start conducting current into the

substrate. The maximum current these diodes can conduct

without causing irreversible damage to the parties is 10 mA.

Capacitor C1 in

Figure 18 is typically about 6 pF and can be

attributed primarily to pin capacitance. Resistor R1 is a lumped

component made up of the on resistance of a switch. This

resistor is typically about 100 Ω. Capacitor C2 is the ADC

sampling capacitor and has a capacitance of 20 pF typically. For

ac applications, removing high frequency components from the

analog input signal is recommended by use of a band-pass filter

on the relevant analog input pin. In applications where

harmonic distortion and signal-to-noise ratio are critical, the

analog input should be driven from a low impedance source.

Large source impedances significantly affect the ac performance

of the ADC. This can necessitate the use of an input buffer

amplifier. The choice of the op amp is a function of the

particular application.

6pF

20pF

CONVERSION PHASE—SWITCH OPEN

TRACK PHASE—SWITCH CLOSED

02976-018

Figure 18. Equivalent Analog Input Circuit

Table 7 provides some typical performance data with various op

amps used as the input buffer for a 100 kHz input tone at room

temperature, under the same setup conditions.

Table 7. AD7920 Typical Performance for Various Input

Buffers, V

= 3 V

Op Amp in the Input Buffer AD7920 SNR Performance (dB)

AD711 72.3

AD797 72.5

AD845 71.4

When no amplifier is used to drive the analog input, the source

impedance should be limited to low values. The maximum

source impedance depends on the amount of total harmonic

distortion (THD) that can be tolerated. The THD increases as

the source impedance increases, and performance degrades (see

Figure 12).

DIGITAL INPUTS

The digital inputs applied to the AD7910/AD7920 are not limited

by the maximum ratings that limit the analog input. Instead, the

digital inputs applied can go to 7 V and are not restricted by the

+ 0.3 V limit as on the analog input. For example, if the

AD7910/AD7920 were operated with a V

of 3 V, then 5 V logic

levels could be used on the digital inputs. However, it is important

to note that the data output on SDATA still have 3 V logic levels

when V

= 3 V. Another advantage of SCLK and

not being

restricted by the V

+ 0.3 V limit is that power supply

sequencing issues are avoided. If

or SCLK is applied before

, there is no risk of latch-up as there would be on the analog

inputs if a signal greater than 0.3 V were applied prior to V

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AD7910ARMZ

Mfr. #:

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Manufacturer:

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Description:

Analog to Digital Converters - ADC 250 kSPS 10-Bit IC

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